**3.1 Laser desorption in modern MS: methods and applications**

As shown in the previous sections, laser induced desorption phenomena play important role in modern MS. The primary role of the laser beam there is to deliver high energy density into some (typically small) volume of the analyte. Due to the local overheating this volume is volatilized forming hot and dense vapor plume, which might be partially ionized. This ionization phenomenon can be considered as a great advantage of laser desorption because there is no need for an additional ionization step, so that the desorbed ions may be directly analyzed by a mass-spectrometer. At the same time, this can be a significant drawback, because, due to collisions in the plume, organic molecules may fragment to the point that their mass analysis becomes meaningless (Miller and Haglund 1998). While using UV lasers in MS analyses of organic materials often produced encouraging results, it is well recognized in the literature that "general mechanism that is applicable to all organic solids

Molecular Desorption by Laser–Driven

Ionizing laser

beam

Desorbing laser beam

Acoustic Waves: Analytical Applications and Physical Mechanisms 355

Because the dominant fraction of the desorbed flux in LIAD are neutral molecules, it is very important to select an appropriate ionization method for the molecules as well as the type of their mass analysis technique. Single-photon ionization (SPI) is well suited for characterization of this phenomenon (Pellin, Calaway et al. 2001) because of its ability to efficiently ionize the desorbing flux with minimal fragmentation. SPI occurs following absorption of a single photon whose energy exceeds the ionization potential (IP) of the molecule of interest, creating a cation. For many molecules, particularly those with aromatic rings to stabilize the cation, fragmentation from photoionization is minimized, and thus the state of the initial LIAD flux can, in principle, be revealed(Lipson and Shi 2002). Currently, the shortest wavelength of commercially available energetic lasers suitable for SPI is 157 nm (F2 laser), which corresponds to the photon energy of 7.9 eV. This energy limits the range of species that can be ionized by SPI to atoms and molecules with IPs less than 7.9 eV. Therefore, organic dyes were chosen as analytes for this LIAD study due to their low IPs, ability to form stable cations, and high photon absorption coefficients. We selected dyes that have IPs in the range from 5 eV to 7 eV and can be easily ionized by the F2-laser radiation.

> Sample carousel

Fig. 5. Schematic drawing of target assembly and laser irradiation pathways. On the right

A time-of-flight mass spectrometer (TOF MS) with a combined LIAD/SPI ion source was employed in our studies of the LIAD phenomenon. The experiments were conducted under ultra high vacuum conditions, with the residual gas pressure in the sample chamber less than 3×10-7 Pa. The schematic drawing of the target assembly in our instrument is shown in Fig. 5. The sample was mounted on one of six sample holders that were supported by a hexagonal carousel. This carousel was driven by an ultrahigh-vacuum compatible motion stage with closed-loop precision of better than 50 nm. The sample holders were secured on the carousel via three 30 mm long alumina ceramic insulators and connected (using vacuum feedthroughs) to a high-voltage pulser unit, which provided voltages necessary for the operating of TOF MS instrument. Each new sample was inserted into the UHV chamber through a vacuum loadlock. Using the motion stage, the sample was then positioned in the focal plane of the TOF MS source optics. The ion optics and operational principles of our instrument are described in more detail elsewhere (Veryovkin, Calaway et al. 2004). A

entrance *S*

*dS* 

Substrate foil

Sample mount

Desorbed molecules cloud

Ion optics

the enlarged view of desorption/ionization scheme is displayed

**3.3 Energy and velocity distributions of desorbed molecules** 

at all UV wavelengths does not exist"(Srinivasan and Braren 1989). The introduction of MALDI gave strong indication that many problems, associated with laser desorption MS might have been solved. However despite popularity of MALDI in MS analyses of proteins, lipids and many other organics (Schiller, Suss et al. 2007), this method cannot be considered as universal because it requires to identify efficient matrix substances for different organic species, and often to develop specialized sample preparation protocols. And, regrettably, MALDI MS cannot be used to directly characterize mixtures of unknown molecules. This is why the search for more versatile and universal methods of molecular desorption/ionization is still on in the analytical mass spectrometry community. From this perspective, the ability of LIAD to volatilize different kinds of organic molecules without noticeable (or, very often, without any) fragmentation has attracted strong interest among researchers.

### **3.2 Laser-induced acoustic desorption**

As described above, the acronym LIAD was suggested in the work conducted by the Chen's team (Golovlev, Allman et al. 1997) where the hypothesis about the acoustic wave nature of the desorption process was expressed. However, LIAD remained just an interesting observation until Kentamaa's team of researchers from Purdue University (Perez, Ramirez-Arizmendi et al. 2000) took on it and demonstrated successful applications of LIAD for the MS analysis of different organic species like cytosine, guanine, thimidine and some others. This work was followed by the series of studies where the applicability of LIAD to the MS analysis of a wide range of organic samples has been demonstrated. The LIAD volatilization method was successfully coupled with Fourier transform ion cyclotron resonance mass spectrometer and alanylglycine (Reid, Tichy et al. 2001), saturated hydrocarbons (Campbell, Crawford et al. 2004), polyethylene (Campbell, Fiddler et al. 2005) and even petroleum distillates (Crawford, Campbell et al. 2005) were analyzed. The great advantages of this technique were the ability to efficiently volatilize various organics and the simplicity of its incorporation into different classes of MS instruments, such as Linear Quadrupole Ion Trap MS (Habicht, Amundson et al. 2010) and Time-of-Flight MS (Zinovev, Veryovkin et al. 2007). Various approaches for the ionization of the desorbed molecules were successfully employed, among them: electron impact and chemical ionization (Crawford, Campbell et al. 2005), single-photon ionization (SPI) (Zinovev, Veryovkin et al. 2007), as well as Elecro- Spray Ionization (ESI) (Cheng, Huang et al. 2009). Moreover, the ability of the LIAD process to non-destructively eject from solid surfaces not only single molecules but also larger intact biological particles, such as viruses (Peng, Yang et al. 2006) and 1 μm size tungsten particles (Menezes, Takayama et al. 2005) have been demonstrated.

In our opinion, the wider spread of LIAD among analytical MS applications is now limited by the lack of an adequate theoretical concept able to explain the existing observations and to predict optimal experimental conditions for future measurements. The mechanical "shake-off" model was proposed only as a qualitative explanation of observed desorption process, and as such was never used to obtain any quantitative agreement between the observable LIAD parameters and the generated acoustic waves. Moreover, to date, there was no work published in the literature, which would be devoted to systematic studies of physical parameters of the molecules desorbed by LIAD. Since we have attempted such as study, we feel it would be beneficial for the research community if we describe here briefly our own experimental methods and experimental results on LIAD.

at all UV wavelengths does not exist"(Srinivasan and Braren 1989). The introduction of MALDI gave strong indication that many problems, associated with laser desorption MS might have been solved. However despite popularity of MALDI in MS analyses of proteins, lipids and many other organics (Schiller, Suss et al. 2007), this method cannot be considered as universal because it requires to identify efficient matrix substances for different organic species, and often to develop specialized sample preparation protocols. And, regrettably, MALDI MS cannot be used to directly characterize mixtures of unknown molecules. This is why the search for more versatile and universal methods of molecular desorption/ionization is still on in the analytical mass spectrometry community. From this perspective, the ability of LIAD to volatilize different kinds of organic molecules without noticeable (or, very often,

As described above, the acronym LIAD was suggested in the work conducted by the Chen's team (Golovlev, Allman et al. 1997) where the hypothesis about the acoustic wave nature of the desorption process was expressed. However, LIAD remained just an interesting observation until Kentamaa's team of researchers from Purdue University (Perez, Ramirez-Arizmendi et al. 2000) took on it and demonstrated successful applications of LIAD for the MS analysis of different organic species like cytosine, guanine, thimidine and some others. This work was followed by the series of studies where the applicability of LIAD to the MS analysis of a wide range of organic samples has been demonstrated. The LIAD volatilization method was successfully coupled with Fourier transform ion cyclotron resonance mass spectrometer and alanylglycine (Reid, Tichy et al. 2001), saturated hydrocarbons (Campbell, Crawford et al. 2004), polyethylene (Campbell, Fiddler et al. 2005) and even petroleum distillates (Crawford, Campbell et al. 2005) were analyzed. The great advantages of this technique were the ability to efficiently volatilize various organics and the simplicity of its incorporation into different classes of MS instruments, such as Linear Quadrupole Ion Trap MS (Habicht, Amundson et al. 2010) and Time-of-Flight MS (Zinovev, Veryovkin et al. 2007). Various approaches for the ionization of the desorbed molecules were successfully employed, among them: electron impact and chemical ionization (Crawford, Campbell et al. 2005), single-photon ionization (SPI) (Zinovev, Veryovkin et al. 2007), as well as Elecro- Spray Ionization (ESI) (Cheng, Huang et al. 2009). Moreover, the ability of the LIAD process to non-destructively eject from solid surfaces not only single molecules but also larger intact biological particles, such as viruses (Peng, Yang et al. 2006) and 1 μm size tungsten particles (Menezes, Takayama et

In our opinion, the wider spread of LIAD among analytical MS applications is now limited by the lack of an adequate theoretical concept able to explain the existing observations and to predict optimal experimental conditions for future measurements. The mechanical "shake-off" model was proposed only as a qualitative explanation of observed desorption process, and as such was never used to obtain any quantitative agreement between the observable LIAD parameters and the generated acoustic waves. Moreover, to date, there was no work published in the literature, which would be devoted to systematic studies of physical parameters of the molecules desorbed by LIAD. Since we have attempted such as study, we feel it would be beneficial for the research community if we describe here briefly

our own experimental methods and experimental results on LIAD.

without any) fragmentation has attracted strong interest among researchers.

**3.2 Laser-induced acoustic desorption** 

al. 2005) have been demonstrated.

## **3.3 Energy and velocity distributions of desorbed molecules**

Because the dominant fraction of the desorbed flux in LIAD are neutral molecules, it is very important to select an appropriate ionization method for the molecules as well as the type of their mass analysis technique. Single-photon ionization (SPI) is well suited for characterization of this phenomenon (Pellin, Calaway et al. 2001) because of its ability to efficiently ionize the desorbing flux with minimal fragmentation. SPI occurs following absorption of a single photon whose energy exceeds the ionization potential (IP) of the molecule of interest, creating a cation. For many molecules, particularly those with aromatic rings to stabilize the cation, fragmentation from photoionization is minimized, and thus the state of the initial LIAD flux can, in principle, be revealed(Lipson and Shi 2002). Currently, the shortest wavelength of commercially available energetic lasers suitable for SPI is 157 nm (F2 laser), which corresponds to the photon energy of 7.9 eV. This energy limits the range of species that can be ionized by SPI to atoms and molecules with IPs less than 7.9 eV. Therefore, organic dyes were chosen as analytes for this LIAD study due to their low IPs, ability to form stable cations, and high photon absorption coefficients. We selected dyes that have IPs in the range from 5 eV to 7 eV and can be easily ionized by the F2-laser radiation.

Fig. 5. Schematic drawing of target assembly and laser irradiation pathways. On the right the enlarged view of desorption/ionization scheme is displayed

A time-of-flight mass spectrometer (TOF MS) with a combined LIAD/SPI ion source was employed in our studies of the LIAD phenomenon. The experiments were conducted under ultra high vacuum conditions, with the residual gas pressure in the sample chamber less than 3×10-7 Pa. The schematic drawing of the target assembly in our instrument is shown in Fig. 5. The sample was mounted on one of six sample holders that were supported by a hexagonal carousel. This carousel was driven by an ultrahigh-vacuum compatible motion stage with closed-loop precision of better than 50 nm. The sample holders were secured on the carousel via three 30 mm long alumina ceramic insulators and connected (using vacuum feedthroughs) to a high-voltage pulser unit, which provided voltages necessary for the operating of TOF MS instrument. Each new sample was inserted into the UHV chamber through a vacuum loadlock. Using the motion stage, the sample was then positioned in the focal plane of the TOF MS source optics. The ion optics and operational principles of our instrument are described in more detail elsewhere (Veryovkin, Calaway et al. 2004). A

Molecular Desorption by Laser–Driven

Acoustic Waves: Analytical Applications and Physical Mechanisms 357

In good agreement with the previously published results on LIAD, we detected strong and stable desorption signals from foils with thickness of 12.5 µm (Fig. 6). Thicker foils (25 µm) produced relatively weak signals for the range of acoustic wave driven laser intensities used in our experiments. For comprehensive experiments, a Ta foil with 12.5 µm thickness was chosen as optimal, not only because high desorption signals were detected from it, but also for its good mechanical strength, high melting point, and durability under powerful laser irradiation. The TOF mass spectra of different organic dye molecules desorbed from the front of the back-irradiated Ta foil surface and ionized by the 157 nm laser radiation are shown in Fig.6. This figure shows three major features, (1) all analytes display large parent molecular ion signals, (2) all spectra display a small number of peaks, with a few or none in the mass range below 100 Da., (3) the number of fragment ion peaks is specific to each molecular analyte. In order to characterize the desorption process in terms of the corresponding molecular fragmentation, a parameter ς, can be defined as a ratio of the sum of intensities of the fragment

ς

BBQ

H13C6 H9 C4

this parameter depends on laser intensities that drive the acoustic waves. For SPI , the parameter ς characterizes not only the peculiarities of the desorption process, but it also generally depends on the photoionization cross-section of the parent molecule and specifics of

its photofragmentation such as possible decay channels and their activation energies.

CH3

N O O CH3

> H CF3

For rhodamine B, the parameter ς grew linearly with KrF laser intensity (Fig. 7), and a similar behavior was also detected for fluorescein, although values of ς were different (ς =0.8 for fluorescein at 300 MW/cm2 of KrF laser intensity). On the contrary, the same experiments conducted for BBQ, surprisingly revealed that the parameter ς decreased with increasing desorption laser power. This discrepancy in ς parameter dependency may indicate that the relationship between these two observations is not a trivial one. This raises the question of whether the desorption and fragmentation phenomena are driven by the same fundamental process. We note that fragmentation is not intrinsic to LIAD. In our experiments, we observed some indications of fragmentation for only three analytes out of five. The analysis of existing data from the literature shows that in some cases fragmentation was observed even with soft ionization of the desorbed molecules whereas in other cases fragmentation was very small or completely absent. A detailed study of

. As will be shown below,

0 200 400 600 800

0 200 400 600 800

399

413

443

Rhodamine B <sup>N</sup> <sup>O</sup> N+

3CH 3CH

520

C4H9 <sup>O</sup>

CH3 CH3

C6H13

<sup>C</sup> <sup>O</sup> OH

675

O

peaks *Af* to the parent molecular peak intensity *Ap* , / <sup>=</sup> *A A f p*

H

H

0 200 400 600 800

Fig. 6. Mass spectra of organic dyes in LIAD experiments

H

Coumarin 281

Anthracene 192

0 200 400 600

dielectric mirror with 98% reflection at 248 nm was mounted in the center of the carousel, in order to deliver the laser beam to the back side of the sample. Note that we will use the convention that the front of the sample is the side facing the ion source and TOF, while the back side is the opposite. For desorption, an excimer KrF laser with wavelength 248 nm (EX10/300 GAM Laser Inc.) was used. The output energy of the laser pulse could be varied between 0.5 - 5 mJ by adjusting the laser discharge voltage and by an additional attenuation with a set of neutral optical filters. The driving laser beam was focused on the target back surface into a spot of rectangular shape ~200×800 μm2 by the fused silica lens with focusing distance of 500 mm. The laser pulse duration was 7 ns, producing a peak power density on the irradiated surface ranging from 50 to 500 MW/cm2. These laser intensities are close to those used in most of LIAD experiments (Perez, Ramirez-Arizmendi et al. 2000; Campbell, Crawford et al. 2004; Campbell, Fiddler et al. 2005; Crawford, Campbell et al. 2005) taking into account that the reflection coefficient in the UV is normally less than it is at visible wavelengths. Post-ionization of the desorbed molecules was performed with an F2 laser, with output energy of 2 mJ/pulse, and pulse duration of 10 ns. The F2 laser beam was focused just above the front target surface, with a waist of 400×2000 μm2, using of a combination of MgF2 spherical and cylindrical lenses. The F2 laser radiation power density in the focal plane was ~10 MW/cm2, which assured the saturation of the photoionization process for the investigated molecules (as verified by a laser power study).

For comparison with LIAD, direct laser desorption (LD) mass spectra were measured for the same samples, also using the F2 laser for post-ionization. To this end, an N2 laser (337 nm wavelength, 100 μJ/pulse energy and 7 ns pulse duration) was focused onto the target front surface using an in-vacuum Schwarzschild optical microscope (Veryovkin, Calaway et al. 2004). The beam spot size on the surface was about 50 μm in diameter.

The delay between the driving KrF (or N2) laser pulses and the ionizing F2 laser could be precisely controlled and varied from 0 to 1000 μs. The desorbed molecules that move away from the surface could therefore be ionized at a precisely defined moment in time and volume in space above the target after the desorption event, with the photoions then analyzed by the TOF MS. This approach allowed us to measure mass spectra for the (postionized) desorbed neutral molecules and determine their velocity distribution. Each mass spectrum was the sum of 128 individual acquired spectra. To prevent the rise of the average foil temperature due to adsorption of laser power, the repetition rate of the laser pulses was maintained at 8 Hz.

Foils from different materials with different thicknesses were used in the experiments. The foil preparation procedure was the same as described in paragraph 2.2.3.1. Before applying the analyte to the top surface of the foil, each substrate was cleaned in methanol-acetone solution (1:1) in an ultrasonic bath (10 minutes).

Organic dyes rhodamine B, fluorescein, methylanthracene (MA), coumarin-522 (N-Methyl-4-trifluoromethylpiperidino3,2-gcoumarin), and BBQ (4,4″-Bisbutyloctyloxy-p-quaterphenyl) were used as received (Eastman Kodak). The dyes were dissolved in methanol (for MA and BBQ, mixed xylenes were also used as solvent), and then the resulting solution (about 10-3 M) was used for sample preparation. One μl of the analyte solution was pipetted onto the foil surface, and then the quartz cylinder-foil assembly was spun at 4500 rpm for 30 seconds to coat the analyte uniformly over the surface. During spin-coating, a significant part of the solution (90% or more) was taken off the surface and, surface concentrations of the analyte could be estimated to be less than 0.5 nM/cm2. After the sample preparation, the foil was introduced into the instrument via the loadlock for analysis.

dielectric mirror with 98% reflection at 248 nm was mounted in the center of the carousel, in order to deliver the laser beam to the back side of the sample. Note that we will use the convention that the front of the sample is the side facing the ion source and TOF, while the back side is the opposite. For desorption, an excimer KrF laser with wavelength 248 nm (EX10/300 GAM Laser Inc.) was used. The output energy of the laser pulse could be varied between 0.5 - 5 mJ by adjusting the laser discharge voltage and by an additional attenuation with a set of neutral optical filters. The driving laser beam was focused on the target back surface into a spot of rectangular shape ~200×800 μm2 by the fused silica lens with focusing distance of 500 mm. The laser pulse duration was 7 ns, producing a peak power density on the irradiated surface ranging from 50 to 500 MW/cm2. These laser intensities are close to those used in most of LIAD experiments (Perez, Ramirez-Arizmendi et al. 2000; Campbell, Crawford et al. 2004; Campbell, Fiddler et al. 2005; Crawford, Campbell et al. 2005) taking into account that the reflection coefficient in the UV is normally less than it is at visible wavelengths. Post-ionization of the desorbed molecules was performed with an F2 laser, with output energy of 2 mJ/pulse, and pulse duration of 10 ns. The F2 laser beam was focused just above the front target surface, with a waist of 400×2000 μm2, using of a combination of MgF2 spherical and cylindrical lenses. The F2 laser radiation power density in the focal plane was ~10 MW/cm2, which assured the saturation of the photoionization

process for the investigated molecules (as verified by a laser power study).

2004). The beam spot size on the surface was about 50 μm in diameter.

pulses was maintained at 8 Hz.

solution (1:1) in an ultrasonic bath (10 minutes).

introduced into the instrument via the loadlock for analysis.

For comparison with LIAD, direct laser desorption (LD) mass spectra were measured for the same samples, also using the F2 laser for post-ionization. To this end, an N2 laser (337 nm wavelength, 100 μJ/pulse energy and 7 ns pulse duration) was focused onto the target front surface using an in-vacuum Schwarzschild optical microscope (Veryovkin, Calaway et al.

The delay between the driving KrF (or N2) laser pulses and the ionizing F2 laser could be precisely controlled and varied from 0 to 1000 μs. The desorbed molecules that move away from the surface could therefore be ionized at a precisely defined moment in time and volume in space above the target after the desorption event, with the photoions then analyzed by the TOF MS. This approach allowed us to measure mass spectra for the (postionized) desorbed neutral molecules and determine their velocity distribution. Each mass spectrum was the sum of 128 individual acquired spectra. To prevent the rise of the average foil temperature due to adsorption of laser power, the repetition rate of the laser

Foils from different materials with different thicknesses were used in the experiments. The foil preparation procedure was the same as described in paragraph 2.2.3.1. Before applying the analyte to the top surface of the foil, each substrate was cleaned in methanol-acetone

Organic dyes rhodamine B, fluorescein, methylanthracene (MA), coumarin-522 (N-Methyl-4-trifluoromethylpiperidino3,2-gcoumarin), and BBQ (4,4″-Bisbutyloctyloxy-p-quaterphenyl) were used as received (Eastman Kodak). The dyes were dissolved in methanol (for MA and BBQ, mixed xylenes were also used as solvent), and then the resulting solution (about 10-3 M) was used for sample preparation. One μl of the analyte solution was pipetted onto the foil surface, and then the quartz cylinder-foil assembly was spun at 4500 rpm for 30 seconds to coat the analyte uniformly over the surface. During spin-coating, a significant part of the solution (90% or more) was taken off the surface and, surface concentrations of the analyte could be estimated to be less than 0.5 nM/cm2. After the sample preparation, the foil was In good agreement with the previously published results on LIAD, we detected strong and stable desorption signals from foils with thickness of 12.5 µm (Fig. 6). Thicker foils (25 µm) produced relatively weak signals for the range of acoustic wave driven laser intensities used in our experiments. For comprehensive experiments, a Ta foil with 12.5 µm thickness was chosen as optimal, not only because high desorption signals were detected from it, but also for its good mechanical strength, high melting point, and durability under powerful laser irradiation. The TOF mass spectra of different organic dye molecules desorbed from the front of the back-irradiated Ta foil surface and ionized by the 157 nm laser radiation are shown in Fig.6. This figure shows three major features, (1) all analytes display large parent molecular ion signals, (2) all spectra display a small number of peaks, with a few or none in the mass range below 100 Da., (3) the number of fragment ion peaks is specific to each molecular analyte. In order to characterize the desorption process in terms of the corresponding molecular fragmentation, a parameter ς, can be defined as a ratio of the sum of intensities of the fragment peaks *Af* to the parent molecular peak intensity *Ap* , / <sup>=</sup> *A A f p* ς . As will be shown below, this parameter depends on laser intensities that drive the acoustic waves. For SPI , the parameter ς characterizes not only the peculiarities of the desorption process, but it also generally depends on the photoionization cross-section of the parent molecule and specifics of its photofragmentation such as possible decay channels and their activation energies.

Fig. 6. Mass spectra of organic dyes in LIAD experiments

For rhodamine B, the parameter ς grew linearly with KrF laser intensity (Fig. 7), and a similar behavior was also detected for fluorescein, although values of ς were different (ς =0.8 for fluorescein at 300 MW/cm2 of KrF laser intensity). On the contrary, the same experiments conducted for BBQ, surprisingly revealed that the parameter ς decreased with increasing desorption laser power. This discrepancy in ς parameter dependency may indicate that the relationship between these two observations is not a trivial one. This raises the question of whether the desorption and fragmentation phenomena are driven by the same fundamental process. We note that fragmentation is not intrinsic to LIAD. In our experiments, we observed some indications of fragmentation for only three analytes out of five. The analysis of existing data from the literature shows that in some cases fragmentation was observed even with soft ionization of the desorbed molecules whereas in other cases fragmentation was very small or completely absent. A detailed study of

Molecular Desorption by Laser–Driven

the same gains of the TOF MS detector.

experimental conditions.

Acoustic Waves: Analytical Applications and Physical Mechanisms 359

mentioned above, time-of-flight mass spectra of such molecules can be measured using the laser post ionization (LPI) technique (Spengler, Bahr et al. 1988). This experimental arrangement makes it possible to determine the distribution of the desorbed neutral molecules over their translational velocities (in the direction normal to the substrate surface) by varying the time delay between desorbing and post-ionizing laser pulses. Raw experimental data in this case are the dependencies of the observed signal on the laser delay time, called below as *signal-vs-delay* dependencies. In order to be able to directly compare LIAD and LD results, the energies of the desorbing N2 (LD) and KrF (LIAD) lasers were adjusted such that the output molecular ion signals reached about the same intensities for

In good agreement with the previous experiments (Perez, Ramirez-Arizmendi et al. 2000), dramatic differences between LIAD and LD in widths of the *signal-vs-delay* dependencies and in positions of their maximums have been observed in these measurements. The most tempting explanation for this experimental finding was that the mean velocities of desorbed molecules in LIAD and LD processes were very different. Unfortunately, no actual energy or velocity distributions of desorbed molecules in LIAD process have been measured experimentally and reported in the literature to date. For the first time, this gap in knowledge can now be filled by processing our experimental data from the *signal-vs-delay* dependencies and converting them into kinetic energy distributions corresponding to our

Our experiment geometry (Fig.5) is rather common, and the detailed discussion of the method as well as the appropriate conversion equations can be found elsewhere (Young, Whitten et al. 1989; Balzer, Gerlach et al. 1997). Typical values of the distance *S* and the thickness of ionization volume *dS* were set to 3 and 0.4 mm, respectively. While, as mentioned above, the value of *S* could be varied in our experiments between 1 mm and 5 mm, in order to achieve optimal compromise between the energy/velocity resolution and the signal-to-noise ratio, most of our data were obtained at *S*=3 mm. At this distance, the relative velocity resolution was *dv/v=dS/S*=0.4/3=0.13 and, correspondingly, the energy resolution was *dE/E*=0.18. A typical velocity distribution obtained for rhodamine B is demonstrated on Fig.10. It is apparent that the desorbed molecules in LIAD are very slow. The average velocities of rhodamine B and BBQ molecules were found to be 59±12 m/s and 47±9 m/s, respectively. For methylanthracene and coumarin522 molecules, the measured average velocities were 76±20 m/s and 70±18 m/s. It is important to recognize that these LIAD-desorbed molecules are much slower than what could be expected assuming the thermal mechanism of LIAD. In order to fit these rhodamine B data with Maxwellian distributions (dashed curve on Fig.10), physically unrealistic low temperatures of about 100 K, were required. On the other hand, the measured velocities are much higher than that of the laser-induced acoustic motion of the foil surface in normal direction, which, measured in the same experimental arrangement, did not exceed 1 m/s. This observation caused serious doubts on the validity of the "shake-off" mechanism considered by many as the most likely cause of LIAD. The measurements of velocity distribution of species desorbed in LD geometry were used for direct comparison with LIAD, and as the proof of validity of our experimental procedure. A smaller insert plot on Fig.10 demonstrates the velocity distribution of desorbed rhodamine B main fragment (399 amu) obtained in the LD irradiation scheme. The mechanisms of the velocity distribution formation in the LD process are well-known and discussed in many reviews (Levis 1994). According to a commonly used procedure described in (Natzle, Padowitz et al. 1988), this distribution can be fit by a

fragmentation in LIAD has not been done yet and the presented here results are the first attempt to quantify this characteristic of LIAD.

Fig. 7. Power dependence of fragmentation parameter

Fig. 8. Power dependence of total desorption yield for rhodamine B molecules

An important characteristic of any desorption phenomenon is its yield. This is why in order to understand the basic processes driving the phenomenon, one has to identify external parameters that have the strongest effect on the desorption yield and then measure a dependence of the yield for each parameter. In the case of LIAD, the dependence of key peak intensities in the mass spectra and the ς parameter on the driving laser intensity appears to be the most important for understanding this phenomenon. The overall desorption yield for all studied analytes strongly increased with desorption laser intensity (within our experimental range), displaying for most peaks approximately exponential dependency. Figure 8 demonstrates this dependence clearly. Plotted on a semi-logarithmic scale, these dependences appear linear, with different slopes for each analyte.

For the characterization of physical nature of the desorption processes, the knowledge of velocity distributions of desorbed neutral molecules is extremely important (Levis 1994). As

fragmentation in LIAD has not been done yet and the presented here results are the first

200 250 300

Desorbing laser intensity, MW/cm2

2.5 3.0 3.5 4.0 4.5 5.0

An important characteristic of any desorption phenomenon is its yield. This is why in order to understand the basic processes driving the phenomenon, one has to identify external parameters that have the strongest effect on the desorption yield and then measure a dependence of the yield for each parameter. In the case of LIAD, the dependence of key peak intensities in the mass spectra and the ς parameter on the driving laser intensity appears to be the most important for understanding this phenomenon. The overall desorption yield for all studied analytes strongly increased with desorption laser intensity (within our experimental range), displaying for most peaks approximately exponential dependency. Figure 8 demonstrates this dependence clearly. Plotted on a semi-logarithmic

For the characterization of physical nature of the desorption processes, the knowledge of velocity distributions of desorbed neutral molecules is extremely important (Levis 1994). As

Fig. 8. Power dependence of total desorption yield for rhodamine B molecules

scale, these dependences appear linear, with different slopes for each analyte.

Desorbing laser energy, mJ

 Rhodamine B Coumarin BBQ

attempt to quantify this characteristic of LIAD.

0.2

Fig. 7. Power dependence of fragmentation parameter

103

104

Signal intensity, a.u.

105

0.3

ζ

0.4

0.5

mentioned above, time-of-flight mass spectra of such molecules can be measured using the laser post ionization (LPI) technique (Spengler, Bahr et al. 1988). This experimental arrangement makes it possible to determine the distribution of the desorbed neutral molecules over their translational velocities (in the direction normal to the substrate surface) by varying the time delay between desorbing and post-ionizing laser pulses. Raw experimental data in this case are the dependencies of the observed signal on the laser delay time, called below as *signal-vs-delay* dependencies. In order to be able to directly compare LIAD and LD results, the energies of the desorbing N2 (LD) and KrF (LIAD) lasers were adjusted such that the output molecular ion signals reached about the same intensities for the same gains of the TOF MS detector.

In good agreement with the previous experiments (Perez, Ramirez-Arizmendi et al. 2000), dramatic differences between LIAD and LD in widths of the *signal-vs-delay* dependencies and in positions of their maximums have been observed in these measurements. The most tempting explanation for this experimental finding was that the mean velocities of desorbed molecules in LIAD and LD processes were very different. Unfortunately, no actual energy or velocity distributions of desorbed molecules in LIAD process have been measured experimentally and reported in the literature to date. For the first time, this gap in knowledge can now be filled by processing our experimental data from the *signal-vs-delay* dependencies and converting them into kinetic energy distributions corresponding to our experimental conditions.

Our experiment geometry (Fig.5) is rather common, and the detailed discussion of the method as well as the appropriate conversion equations can be found elsewhere (Young, Whitten et al. 1989; Balzer, Gerlach et al. 1997). Typical values of the distance *S* and the thickness of ionization volume *dS* were set to 3 and 0.4 mm, respectively. While, as mentioned above, the value of *S* could be varied in our experiments between 1 mm and 5 mm, in order to achieve optimal compromise between the energy/velocity resolution and the signal-to-noise ratio, most of our data were obtained at *S*=3 mm. At this distance, the relative velocity resolution was *dv/v=dS/S*=0.4/3=0.13 and, correspondingly, the energy resolution was *dE/E*=0.18. A typical velocity distribution obtained for rhodamine B is demonstrated on Fig.10. It is apparent that the desorbed molecules in LIAD are very slow. The average velocities of rhodamine B and BBQ molecules were found to be 59±12 m/s and 47±9 m/s, respectively. For methylanthracene and coumarin522 molecules, the measured average velocities were 76±20 m/s and 70±18 m/s. It is important to recognize that these LIAD-desorbed molecules are much slower than what could be expected assuming the thermal mechanism of LIAD. In order to fit these rhodamine B data with Maxwellian distributions (dashed curve on Fig.10), physically unrealistic low temperatures of about 100 K, were required. On the other hand, the measured velocities are much higher than that of the laser-induced acoustic motion of the foil surface in normal direction, which, measured in the same experimental arrangement, did not exceed 1 m/s. This observation caused serious doubts on the validity of the "shake-off" mechanism considered by many as the most likely cause of LIAD. The measurements of velocity distribution of species desorbed in LD geometry were used for direct comparison with LIAD, and as the proof of validity of our experimental procedure. A smaller insert plot on Fig.10 demonstrates the velocity distribution of desorbed rhodamine B main fragment (399 amu) obtained in the LD irradiation scheme. The mechanisms of the velocity distribution formation in the LD process are well-known and discussed in many reviews (Levis 1994). According to a commonly used procedure described in (Natzle, Padowitz et al. 1988), this distribution can be fit by a

Molecular Desorption by Laser–Driven

the power dependence.

Acoustic Waves: Analytical Applications and Physical Mechanisms 361

measured for these molecules are shown on Fig.11. Dashed line represents Maxwell distribution for rhodamine B corresponding to the 80 K temperature. This temperature was chosen to obtain the best fit at the distributions maximums. At the same time, high energy tails of the experimental energy distributions strongly deviate from the exponential law, which is apparent with the double logarithmic scale in Fig.11, and reveal behavior close to

> Anthracene Rhodamine B (HE) Rhodamine B (LE)

> > τ

 α= *l* / ,

 BBQ Coumarin

10-4 10-3 10-2 10-1

The "shake-off" mechanism of molecular desorption in LIAD for a long time was considered as the only sensible explanation of the observed phenomenon. However, as was shown above, it contradicts both with general physical considerations and with the experimentally measured parameters of acoustic vibrations of the surface as well as with the observed

An important consequence of the backside irradiation is the heating of the front side. Considering this effect, we should keep in mind that this heating process in LIAD is distinctly different from the case of direct front-side laser irradiation (LD). In the latter case, due to the small value of skin depth in metals, the rate of the temperature rise is extremely high. In case of backside irradiation of the foils, the front surface temperature is governed by the heat conduction through the metal foil, which makes the heating rate much slower than that for the front side irradiation geometry. For one-dimensional heat conduction problem, the specific time of the temperature rise is defined by the heat propagation time <sup>2</sup>

where *α* is the thermal diffusivity of the metal and *l* is the foil thickness. For the foil thicknesses typical for LIAD (~10 µm) and the thermal diffusivity of the most metals *α*≈0.2 – 0.7 cm2s-1, *τ* has the values in the range of a few µs. Our numerical calculations using the heat conduction equation showed that for Ta foil with the thickness of 12.5 µm and the driving laser fluence 3.5 J/cm2 (corresponding under conditions of our experiments to the peak laser intensity of about 500 MW/cm2), the front surface temperature rise is 375 K, which reaches its maximum 1.75 µs after the laser desorbing pulse ceases. If experiments

Fig. 11. Energy distribution of different organic dyes molecules. Dashed line represents

**4. Is desorption process in LIAD really driven by acoustic waves? 4.1 "Shake-off "mechanism versus thermal origin of molecular desorption** 

Energy, eV

10-6 10-5 10-4 10-3 10-2 10-1 100

equilibrium Boltzmann distribution at T=100 K

energy and velocity spectra of desorbed molecules.

dN/dE, normalized

two-temperature bi-modal velocity distribution: dashed lines in the insert plot represent spectral components with different temperatures, and the solid line corresponds to their sum. Being in a good agreement with general LD regularities, these results also confirm the validity of the experimental procedure used for both LD and LIAD.

Fig. 9. The comparison of *signal-vs-delay* for LIAD and LD desorbed rhodamine B molecules

Velocity distributions were measured also at different fluences of the driving KrF laser. On Fig. 10, two distributions corresponding to laser fluences of 2.3 J/cm2 and 3.4 J/cm2 are plotted. Within the limits of accuracy of our measurements, no change of average velocity has been detected, which suggests that both thermal and "shake-off" models are not applicable for the explanation of LIAD process.

Fig. 10. Velocity distribution for rhodamine B molecules at different LIAD desorption laser intensities. On insert there is the same distribution for LD regime

In order to calculate the mean energies of desorbed molecules, we used the described above approach and took into account that the Jacobian of this variable transformation was given by *S*2*/t*3 (Balzer, Gerlach et al. 1997). The mean energies of molecules in LIAD experiments (as well as their mean velocities) showed no apparent trend with the increase of desorbing laser fluences keeping the average value at about 9 meV for rhodamine B, 9.5 meV for BBQ, 6.5meV for coumarine 522 and 7.5meV for methylanthracene. Typical energy distributions

two-temperature bi-modal velocity distribution: dashed lines in the insert plot represent spectral components with different temperatures, and the solid line corresponds to their sum. Being in a good agreement with general LD regularities, these results also confirm the

Fig. 9. The comparison of *signal-vs-delay* for LIAD and LD desorbed rhodamine B molecules Velocity distributions were measured also at different fluences of the driving KrF laser. On Fig. 10, two distributions corresponding to laser fluences of 2.3 J/cm2 and 3.4 J/cm2 are plotted. Within the limits of accuracy of our measurements, no change of average velocity has been detected, which suggests that both thermal and "shake-off" models are not

1.25 Rhodamine D, LE

Rhodamine B, HE

0 500 1000

Velocity, m/s

0 50 100 150 200 250 300

0.0

dN/dV, rel.units

Velocity, m/s

Fig. 10. Velocity distribution for rhodamine B molecules at different LIAD desorption laser

In order to calculate the mean energies of desorbed molecules, we used the described above approach and took into account that the Jacobian of this variable transformation was given by *S*2*/t*3 (Balzer, Gerlach et al. 1997). The mean energies of molecules in LIAD experiments (as well as their mean velocities) showed no apparent trend with the increase of desorbing laser fluences keeping the average value at about 9 meV for rhodamine B, 9.5 meV for BBQ, 6.5meV for coumarine 522 and 7.5meV for methylanthracene. Typical energy distributions

validity of the experimental procedure used for both LD and LIAD.

applicable for the explanation of LIAD process.

0.00

intensities. On insert there is the same distribution for LD regime

0.25

0.50

dN/dV, normalized

0.75

1.00

measured for these molecules are shown on Fig.11. Dashed line represents Maxwell distribution for rhodamine B corresponding to the 80 K temperature. This temperature was chosen to obtain the best fit at the distributions maximums. At the same time, high energy tails of the experimental energy distributions strongly deviate from the exponential law, which is apparent with the double logarithmic scale in Fig.11, and reveal behavior close to the power dependence.

Fig. 11. Energy distribution of different organic dyes molecules. Dashed line represents equilibrium Boltzmann distribution at T=100 K
