**2.2.3 Experimental observations of laser-generated acoustic waves in thin foils**

Experimental studies of acoustic waves in solids due to pulsed laser irradiation have started with the advent of such lasers (White 1963). A great collection of experimental results and theoretical analyses of acoustic wave generation in solids driven by laser pulses has been accumulated since (Hutchins 1985), and these studies continue at present (Xu, Feng et al. 2008). Regrettably, there is a very limited data set, which could be used to interpret of LIAD experiments. To prove (or disapprove) the "shake-off" hypothesis of molecular desorption, direct measurements of thin foil surface velocities in back-side irradiation geometry are required. The scarcity of such data motivated us to setup a series of our own experiments aiming at measurements of thin foils vibrations under typical LIAD conditions. Experimental approaches to this problem are well known and described in the literature (Scruby and Wadley 1978; Royer and Dieulesaint 2000). Nevertheless, we will briefly describe below our system, in order to create a better stage for presentation and discussion of the original results.

### **2.2.3.1 Experimental technique: optical and electrical methods**

One of the most popular and widely used methods to studies of acoustic waves is based on non-contact optical measurements. Figure 1a shows the experimental setup for measurements of surface displacement using interferometry-based approach. A He-Ne laser (1) (Melles-Griot, 543 nm, 0.5 mW) was used as the light source for a Michelson interferometer. It consisted of a beam splitter (5), an etalon and steering mirrors (3, 4), a focusing lens (6), a target (7), an imaging lens (9), an aperture (13), a focusing lens (14) and a photomultiplier (15). The target was back-irradiated by a pulsed laser (12) through a fused silica lens (8). Laser beam parameters were measured by intersecting the laser flux with two

Molecular Desorption by Laser–Driven

transducer-based sensor was about 5 nm.

a planar capacitor

Acoustic Waves: Analytical Applications and Physical Mechanisms 351

similar to that described in Ref.19. The pin was connected to the input of a miniature charge amplifier (7) powered by a constant (20 mA) direct current (DC) supply (8) and connected to the oscilloscope (10). The bandwidth of the charge amplifier was about 2 MHz, which provided a signal rise time of less than 400 ns. To increase the overall sensitivity of the

The sensitivity of the transducer to surface displacement can easily be expressed in terms of

2 <sup>−</sup> Δ= Δ *VS q d <sup>d</sup>* ε

where Δ*q* is the change in the electric charge, *V* is the applied voltage, *S* is the surface area of the pin tip, *d* is the width of the gap between the electrodes and *Δd* is the change of the width. The sensitivity of the charge preamplifier was 10 mV/pC, which corresponded to a minimal detectable signal of about 5 mV (thus yielding reasonably good signal-to-noise ratio). With an applied bias potential of -100 V, the estimated detection limit of the

Foils with various thicknesses (from 12.5 μm up to 100 µm) made from different materials were used in our experiments. Materials were selected to span the range from soft metals (Au, Al and Ni) to refractory metals (W, Mo and Ta) and semiconductors (Si). For each experiment, the front surface of the sample was mechanically polished to roughness of less than 0.250 µm (RMS). After polishing, the foils were glued with silver epoxy to the rim of a hollow quartz cylinder (8 mm outside diameter, 8 mm height, and 0.5 mm wall thickness). The epoxy was cured for 2 hours in an oven at temperature of 100º C. Due to the differences between thermal expansion coefficients of the foil materials and quartz, the foil stretched over the top of the quartz cylinder once the assembly cooled to room temperature. The tension was not very strong (according to our estimates, the total radial force did not exceed 1 N), and therefore, the foils in our experiments may be considered as supported at the edges. The silver epoxy also provided a conductive path between the sample and the

Lasers generating both ultraviolet (UV) and infrared (IR) light were used for target irradiation. The UV light was generated by an ArF excimer laser (EX10-300, GAM, Inc.) having a wavelength 193 nm and a pulse duration of 15 ns. The output pulse energy could be varied from 0.4 to 4 mJ by using neutral density optical filters. The laser radiation was focused onto the backside of the target (opposite to the surface displacement sensors) using a fused silica lens with a focusing distance of f=300 mm. The irradiated spot on the target had rectangular dimensions of 100×500 μm, which corresponded to a UV laser power density in the range of 50–500 MW/cm2. For IR irradiation of the target, a Q-switched Nd:YAG laser (Continuum) was used. This IR light had a wavelength of 1064 nm, a 12 ns pulse duration and an output energy in the range of 1–15 mJ/pulse. The IR laser beam had a Gaussian profile and produced a spot on the target surface with a nominal diameter of 500

Waveforms representing foil oscillations were measured over the time range from 5 μs to 5 ms. For times much greater than the laser pulse duration (t>>τ), a decaying quasi-harmonic oscillations were observed for all materials. The measured time dependence of the displacement of the foils irradiated by laser pulses with different intensities exhibited

**2.2.3.2 Experimental results: displacement and surface velocity of thin foils** 

instrument by placing a silver epoxy track along the quartz cylinder side.

μm, corresponding to peak power density of 40–600 MW/cm2.

(15)

detector, a positive bias potential of 100 V was applied to the target.

partially reflecting (8%) quartz plates (10, 11) directing reflected beams onto a fast photodiode (16) and an energy meter (17), respectively. A wedge-type optical attenuator (18) was used to balance the Michelson interferometer shoulders and to increase the contrast of the resulting interference pattern. Focusing lenses (6) and (8) were mounted on three-axis translation stages. In this arrangement, both the acoustic-wave generating and diagnostic laser beams could be independently focused and translated to different points on the target surface. A lens (9) formed the magnified interference pattern in the plane of an aperture (13) whose diameter was selected to be equal to the width of the dark band of the lowest interference order. A second lens (14) was used to collect the light, which passed through the aperture (13), and to direct it to the photomultiplier (PMT, 15).

The anode of the PMT was terminated with a 50 Ohm load to allow optical signals with rise times as short as 5 ns to be measured. This capability was confirmed by demonstrating that the shape of a 12 ns, NdYAG laser pulse was identical when measured by using this detection system and by a high-speed avalanche photodiode. The measurements bandwidth was limited by the PicoScope 3206 oscilloscope (200 MHz bandwidth and 200 Ms/s sampling rate), which was used for signal acquisition. The digitized PMT signal was transmitted to a PC via USB port and stored for further processing. The oscilloscope was triggered by a pulse from the fast photodiode (17). The measured lag between the trigger signal and the PMT signal was less than 40 ns. The maximum signal amplitude, corresponding to the peak-to-valley ratio of the interference pattern, was 80 mV. The minimum detectable signal was 5 mV at signal-to-noise ratio of about 3, which corresponds to a surface displacement of approximately 25 nm. However, because of the strong electrical noise generated by the Q-switch of the laser, the smallest surface displacement detectable in this series of experiments was about 40 nm.

Fig. 1(a,b). Schematic drawing of the experimental setup for laser-driven acoustic wave studies. (a) Interferometry method; (b) Capacitance method

The same target irradiation scheme as described above was also used for the capacitance transducer measurements (Fig.1b). In contrast to the previous approach, a metal pin was placed in front of the target. This pin served as the second plate of a capacitor whose first plate was the target. These two capacitor electrodes were separated by a small gap *d*, typically about 100 μm. Both the target and the pin were fixed in optical mounts that allowed alignment in the plane of the sample surface. In addition, the mount of the pin was placed on a translation stage (9), driven by a picomotor, which could move the target (with the precision of 1 µm) in the direction orthogonal to the target surface. The pin had a diameter of 3 mm and its end was polished flat. In general, the design of this detector is

partially reflecting (8%) quartz plates (10, 11) directing reflected beams onto a fast photodiode (16) and an energy meter (17), respectively. A wedge-type optical attenuator (18) was used to balance the Michelson interferometer shoulders and to increase the contrast of the resulting interference pattern. Focusing lenses (6) and (8) were mounted on three-axis translation stages. In this arrangement, both the acoustic-wave generating and diagnostic laser beams could be independently focused and translated to different points on the target surface. A lens (9) formed the magnified interference pattern in the plane of an aperture (13) whose diameter was selected to be equal to the width of the dark band of the lowest interference order. A second lens (14) was used to collect the light, which passed through

The anode of the PMT was terminated with a 50 Ohm load to allow optical signals with rise times as short as 5 ns to be measured. This capability was confirmed by demonstrating that the shape of a 12 ns, NdYAG laser pulse was identical when measured by using this detection system and by a high-speed avalanche photodiode. The measurements bandwidth was limited by the PicoScope 3206 oscilloscope (200 MHz bandwidth and 200 Ms/s sampling rate), which was used for signal acquisition. The digitized PMT signal was transmitted to a PC via USB port and stored for further processing. The oscilloscope was triggered by a pulse from the fast photodiode (17). The measured lag between the trigger signal and the PMT signal was less than 40 ns. The maximum signal amplitude, corresponding to the peak-to-valley ratio of the interference pattern, was 80 mV. The minimum detectable signal was 5 mV at signal-to-noise ratio of about 3, which corresponds to a surface displacement of approximately 25 nm. However, because of the strong electrical noise generated by the Q-switch of the laser, the smallest surface displacement detectable in

the aperture (13), and to direct it to the photomultiplier (PMT, 15).

this series of experiments was about 40 nm.

6

9

7

3

5

1 2

4 13 14

studies. (a) Interferometry method; (b) Capacitance method

8

10 11 12

18 *d* 

16 17

15

The same target irradiation scheme as described above was also used for the capacitance transducer measurements (Fig.1b). In contrast to the previous approach, a metal pin was placed in front of the target. This pin served as the second plate of a capacitor whose first plate was the target. These two capacitor electrodes were separated by a small gap *d*, typically about 100 μm. Both the target and the pin were fixed in optical mounts that allowed alignment in the plane of the sample surface. In addition, the mount of the pin was placed on a translation stage (9), driven by a picomotor, which could move the target (with the precision of 1 µm) in the direction orthogonal to the target surface. The pin had a diameter of 3 mm and its end was polished flat. In general, the design of this detector is

Fig. 1(a,b). Schematic drawing of the experimental setup for laser-driven acoustic wave

8

1 2 3, 3'

11, 12

4

5 6 7

10

9

similar to that described in Ref.19. The pin was connected to the input of a miniature charge amplifier (7) powered by a constant (20 mA) direct current (DC) supply (8) and connected to the oscilloscope (10). The bandwidth of the charge amplifier was about 2 MHz, which provided a signal rise time of less than 400 ns. To increase the overall sensitivity of the detector, a positive bias potential of 100 V was applied to the target.

The sensitivity of the transducer to surface displacement can easily be expressed in terms of a planar capacitor

$$
\Delta q = \frac{-\varepsilon VS}{d^2} \Delta d \tag{15}
$$

where Δ*q* is the change in the electric charge, *V* is the applied voltage, *S* is the surface area of the pin tip, *d* is the width of the gap between the electrodes and *Δd* is the change of the width. The sensitivity of the charge preamplifier was 10 mV/pC, which corresponded to a minimal detectable signal of about 5 mV (thus yielding reasonably good signal-to-noise ratio). With an applied bias potential of -100 V, the estimated detection limit of the transducer-based sensor was about 5 nm.

### **2.2.3.2 Experimental results: displacement and surface velocity of thin foils**

Foils with various thicknesses (from 12.5 μm up to 100 µm) made from different materials were used in our experiments. Materials were selected to span the range from soft metals (Au, Al and Ni) to refractory metals (W, Mo and Ta) and semiconductors (Si). For each experiment, the front surface of the sample was mechanically polished to roughness of less than 0.250 µm (RMS). After polishing, the foils were glued with silver epoxy to the rim of a hollow quartz cylinder (8 mm outside diameter, 8 mm height, and 0.5 mm wall thickness). The epoxy was cured for 2 hours in an oven at temperature of 100º C. Due to the differences between thermal expansion coefficients of the foil materials and quartz, the foil stretched over the top of the quartz cylinder once the assembly cooled to room temperature. The tension was not very strong (according to our estimates, the total radial force did not exceed 1 N), and therefore, the foils in our experiments may be considered as supported at the edges. The silver epoxy also provided a conductive path between the sample and the instrument by placing a silver epoxy track along the quartz cylinder side.

Lasers generating both ultraviolet (UV) and infrared (IR) light were used for target irradiation. The UV light was generated by an ArF excimer laser (EX10-300, GAM, Inc.) having a wavelength 193 nm and a pulse duration of 15 ns. The output pulse energy could be varied from 0.4 to 4 mJ by using neutral density optical filters. The laser radiation was focused onto the backside of the target (opposite to the surface displacement sensors) using a fused silica lens with a focusing distance of f=300 mm. The irradiated spot on the target had rectangular dimensions of 100×500 μm, which corresponded to a UV laser power density in the range of 50–500 MW/cm2. For IR irradiation of the target, a Q-switched Nd:YAG laser (Continuum) was used. This IR light had a wavelength of 1064 nm, a 12 ns pulse duration and an output energy in the range of 1–15 mJ/pulse. The IR laser beam had a Gaussian profile and produced a spot on the target surface with a nominal diameter of 500 μm, corresponding to peak power density of 40–600 MW/cm2.

Waveforms representing foil oscillations were measured over the time range from 5 μs to 5 ms. For times much greater than the laser pulse duration (t>>τ), a decaying quasi-harmonic oscillations were observed for all materials. The measured time dependence of the displacement of the foils irradiated by laser pulses with different intensities exhibited

Molecular Desorption by Laser–Driven

Acoustic Waves: Analytical Applications and Physical Mechanisms 353

 100 MW/cm 190 MW/cm 320 MW/cm 430 MW/cm 510 MW/cm


Fig. 3. Time dependent displacement of Ta foil (12.5 μm thick) at different laser intensities

Time, s

0.0 5.0x10-6 1.0x10-5 1.5x10-5

Time, s Fig. 4. Time dependence of Ta foil (12.5 mm thick) surface velocity at 400 MW/cm2 driving

This picture clearly confirms the statement above that the mass transfer velocity (or surface displacement velocity, in terms of our experiment) is much slower than the speed of sound in metals. In its turn, this result supports our hypothesis that the vibrational motion of the foil surface cannot serve as direct cause of molecular desorption, and that *the real physical* 

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

**3. Desorption of the molecules from back-irradiated thin metal foils** 

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

Displacement, nm


*mechanism of LIAD is not as simple as mechanical shake-off*.

Velocity, m/s

laser intensity

qualitatively similar behavior, although amplitudes, frequencies and decay times varied. These results suggest that each foil behave as a mechanical system able to oscillate in a freerunning mode after the external force is removed. Fast Fourier Transform (FFT) analysis applied to the measured data has shown that the frequency spectra consisted of discrete lines (modes) appearing in the range of 10–100 kHz.

In contrast to the steady-state regime (*t>>τ)* when different foils oscillated very similarly, the initial moment (*t<τ*) of the evolving oscillation was distinctly different for each foil. Fig. 2 presents the time dependence of the surface displacement for different metals at low irradiation intensities (50 MW/cm2) for "early" times in the range up to 50 μs. For all measurements at low intensities, the initial displacement is found to be negative, indicating that the surface is first depressed (i.e. towards to the driving laser beam and, correspondingly, away from the detector). Increasing the laser intensity leads to the initial surface vibration waveform displacement changing from negative to positive. This is due to the recoil pulse which occurs when material is ablated from the irradiated surface due to plasma formation. Fig. 3 shows the time dependences of the displacement of Ta foil surface for different laser irradiation intensities. The plasma formation threshold for this Ta foil was ~220 MW/cm2, as determined by the observation of the plasma plume glow in a separate experiment. The FFT analysis of the experimental data at higher laser intensities revealed that the frequency spectrum of the foil oscillations in the non-steady-state regime contains much higher frequency components than found for the steady-state case. After a few tens of microseconds, the high frequency components disappear as the foil oscillation become harmonic as described by Eq.(4).

Fig. 2. Time dependent displacement for different metal foils at low laser intensities (50 MW/cm2

The obtained results are in good agreement with time dependencies of surface displacement measured under slightly different experimental conditions (Hutchins 1985). This fact clearly demonstrates that laser-driven acoustic waves in thin metal foils have no experimental peculiarities distinguishing them from well-known acoustic wave mechanisms. This result allowed us to calculate surface velocities using the measured surface displacements (Fig. 4). As one can see from Fig.4, these velocities are indeed in the range of meters per second.

qualitatively similar behavior, although amplitudes, frequencies and decay times varied. These results suggest that each foil behave as a mechanical system able to oscillate in a freerunning mode after the external force is removed. Fast Fourier Transform (FFT) analysis applied to the measured data has shown that the frequency spectra consisted of discrete

In contrast to the steady-state regime (*t>>τ)* when different foils oscillated very similarly, the initial moment (*t<τ*) of the evolving oscillation was distinctly different for each foil. Fig. 2 presents the time dependence of the surface displacement for different metals at low irradiation intensities (50 MW/cm2) for "early" times in the range up to 50 μs. For all measurements at low intensities, the initial displacement is found to be negative, indicating that the surface is first depressed (i.e. towards to the driving laser beam and, correspondingly, away from the detector). Increasing the laser intensity leads to the initial surface vibration waveform displacement changing from negative to positive. This is due to the recoil pulse which occurs when material is ablated from the irradiated surface due to plasma formation. Fig. 3 shows the time dependences of the displacement of Ta foil surface for different laser irradiation intensities. The plasma formation threshold for this Ta foil was ~220 MW/cm2, as determined by the observation of the plasma plume glow in a separate experiment. The FFT analysis of the experimental data at higher laser intensities revealed that the frequency spectrum of the foil oscillations in the non-steady-state regime contains much higher frequency components than found for the steady-state case. After a few tens of microseconds, the high frequency components disappear as the foil oscillation become


 Ni Ta Au Al

Time, s

The obtained results are in good agreement with time dependencies of surface displacement measured under slightly different experimental conditions (Hutchins 1985). This fact clearly demonstrates that laser-driven acoustic waves in thin metal foils have no experimental peculiarities distinguishing them from well-known acoustic wave mechanisms. This result allowed us to calculate surface velocities using the measured surface displacements (Fig. 4). As one can see from Fig.4, these velocities are indeed in the range of meters per second.

Fig. 2. Time dependent displacement for different metal foils at low laser intensities (50

lines (modes) appearing in the range of 10–100 kHz.

harmonic as described by Eq.(4).

MW/cm2


Displacement, nm

Fig. 3. Time dependent displacement of Ta foil (12.5 μm thick) at different laser intensities

Fig. 4. Time dependence of Ta foil (12.5 mm thick) surface velocity at 400 MW/cm2 driving laser intensity

This picture clearly confirms the statement above that the mass transfer velocity (or surface displacement velocity, in terms of our experiment) is much slower than the speed of sound in metals. In its turn, this result supports our hypothesis that the vibrational motion of the foil surface cannot serve as direct cause of molecular desorption, and that *the real physical mechanism of LIAD is not as simple as mechanical shake-off*.
