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Comparison of two methods of UV laser-induced surface ionization
Volume 7 l, number 3,4
OPTICS COMMUNICATIONS
15 May 1989
C O M P A R I S O N O F T W O M E T H O D S O F UV L A S E R - I N D U C E D SURFACE I O N I Z A T I O N Mo Y A N G and James P. REILLY Indiana University, Chemistry Department, Bloomington, IN 4 7405, USA
Received 29 December 1988
Two different methods of UV laser-induced desorption ionization are compared by measuring the dependence of each on the laser pulse repetition rate. At an aniline sample pressure of 2.3 × l0 -s torr, the ionization signal obtained with a prism internal reflection geometry increases linearly with the time between laser pulses. In contrast, the signal obtained with light incident on the surface from the gas phase side is independent of the laser repetition rate, as in conventional gas phase ionization experiments. These observations, in conjunction with measurements of the time of flight peak distribution, enable us to ascertain the locations where ionization takes place in the two experiments. UV laser-induced surface ionization with prism internal reflection is found to be appropriate for studying adsorption/desorption kinetics in experiments where gas phase molecules are continuously adsorbing and desorbing from a metal surface.
1. Introduction One approach to monitoring molecules adsorbed on a metal surface is to b o m b a r d the surface with particles or light, desorb the adsorbate, and then ionize the molecules that have been ejected into the gas phase [ 1,2 ]. The latter step can be performed with electrons or through a resonant gas phase laser ionization process. By using relatively low incident light intensities, temperatures increments and damage at the surface can be minimized [ 3 ]. In a few recent studies, we have been investigating the mechanism of ionization that takes place when low intensity U V laser pulses b o m b a r d thin metal films that have been coated on fused silica substrates and mounted in the source region o f a time o f flight mass spectrometer [ 4 ]. Volatile aromatic molecules that have been introduced into the gas phase above the metal film adsorb on it to some extent and are probed by the incident laser pulse. We previously demonstrated that when the laser beam is incident upon the metal film from the gas phase side at angles approaching grazing incidence, an interference pattern is generated above the surface whose periodicity is much larger than the light wavelength. Molecules present in the gas phase at positions corresponding to the antinodes o f the radiation's electric field can be excited
and ionized, leading to time of flight mass spectra that exhibit multiple peaks [ 5 ]. Although we have often observed these mass spectral patterns, it has not been established whether the first peak in a multiple peak distribution results from the ionization o f gas phase or surface-adsorbed molecules. F r o m a close examination o f the time o f flight distributions obtained in this "external" surface irradiation mode this will now be clearly elucidated. In order to avoid the multiple gas phase ionization peaks in our time o f flight mass spectra, we previously showed that a thin metal film can be irradiated by UV light pulses incident from the substrate side and the resulting evanescent wave has sufficient intensity to excite and ionize molecules adsorbed to the film [ 6 ]. In this case, gas phase ionization at significant distances above the surface is repressed since the radiation's interference pattern must be contained within the substrate, and the evanescent light wave does not project far into the gas phase [ 7 ]. Although in the one case o f N O adsorbed onto a gold film the wavelength dependence o f the process indicated that laser-induced desorption precedes gas phase ionization [4], this result may not be general. However, we have suggested that those molecules that are ionized in this "internal reflection" mode must be adsorbed on the surface at least at the onset o f the 193
Volume 71, number 3,4
OPTICS COMMUNICATIONS
light pulse. Since the number of molecules adsorbed on our metal film surface depends on the equilibrium between adsorption and desorption processes, and since incident laser light increases the rate of molecular desorption, we have investigated the dependence of the observed ion yield on laser pulse repetition rate. The rather striking results to be presented demonstrate that this is an effective method of distinguishing sample molecules adsorbed on a surface from those moving about in the gas phase above it.
2. Experimental The apparatus used to record the laser ionization mass spectra of aromatic molecules in either the "external" or "internal" modes as defined above has been described previously [4 ]. Briefly, a thin film of gold was coated onto the hypotenuse of a right angle prism that was mounted in the source region of a UHV TOF laser ionization mass spectrometer. The gold film's thickness was estimated by a resistance measurement to be about 0.5 to l nm [8]. 260 nm UV laser pulses generated by frequency doubling the output of a tunable dye laser irradiated the metal film from either the gas phase or fused silica side. The 1 microjoule light pulses were typically l to 2 nanoseconds in duration and were focussed to a diameter of about 0.1 mm at the metal surface. Since one of the principal experiments involved varying the laser repetition rate while keeping all other parameters constant, it was necessary to devise an arrangement that maintained a constant laser output energy and beam profile as its repetition rate changed by a factor of 30. This was accomplished by operating the laser at a constant repetition rate of 20 Hz and by employing a mechanical chopper that transmitted light pulses at selected intervals. With this arrangement repetition rates between 0.8 and 20 Hz were easily accessible. Aniline vapor at a pressure of 2.3 X l0 -8 torr was maintained in the gas phase above the metal surface. Laser-generated parent ions of mass 93 daltons were accelerated by a high ion drawout field (2000 volt across a 6.2 mm gap) toward a microchannel plate detector. The detector output was amplified, digitized and stored in an IBM/AT computer. Typical 194
15 May 1989
experiments involved signal averaging the results of 1000 light pulses. Fig. i displays the external and internal geometries that were used to record laser ionization mass spectra. Experimental data, in the form of a multiple peak mass spectrum, obtained with the former geometry, and a single sharp peak, obtained the the latter, are also displayed. (The small, second peak in the latter mass spectrum arises from re~z=94 ions present due to the natural abundance of ~ac.) In order to ascertain whether the first peak in fig. lb results from ionization of aniline at the gold film surface or at the first antinode of the light interference pattern above the surface we recorded multiple peak mass spectra at near-grazing angles of incidence, 0, equal to 0.28 and 0.62 °. The spectrometer fields and all other experimental parameters were held constant. The resulting mass spectra are displayed in fig. 2. It is clear that the first peaks in the two mass spectra do not occur at the same point in time, indicating that they do not arise from ionization of surface-adsorbed molecules. In fact, the Fresnel equations indicate that a node of the electric field should occur at the metal/gas boundary and the first antinode should be at a height above the surface equal to half of the periodicity of the interference pattern. This implies that an ion formed at the surface with zero initial kinetic energy should have a flight time given by the location of the dotted lines in figs. 2a and 2b. a) TOF MS
6
c) TOF MS
t
s'o
~6o
1~o
z6o
d l 6
s'o ~oo ~o 260 Channel Number [lch=2ns]
Fig. I. Comparison of two different UV laser-induced surface ionization methods. (a), (b) External reflection geometryand measured mass spectrum. (c), (d) Internal reflection geometry and measured mass spectrum.
Volume 71 number 3,4
OPTICS COMMUNICATIONS
0 I
20 i
i
40 n I
'/I//~ i
so
60 i
I
80 n
I
g
100 Jam I
I
: 0,2B*
40
1~0
b) i
' 50
0
= 0,62"
100
l~aO
Channet Number [lch=2ns]
Fig. 2. Laser ionization TOF mass spectra obtained in the external reflection geometry and displayed on an expanded time scale for two different anglesof incidence. (a) 0=0.28% (b) 0=0.62 °. The calculated position scale is indicated at the top.
the ion yield per laser shot is independent of the laser pulse repetition rate. Based on our interpretation o f the multiple peak mass spectra as described above, one would expect that their intensity would also be independent o f the laser shot interval. In fig. 3 we demonstrate that both the total area o f all o f the multiple peaks and the area o f the first o f the multiple peaks are independent o f laser shot interval. In contrast, the area of the single sharp peak generated in the internal ionization geometry is seen to be highly dependent on the repetition rate of the laser. The dependence appears to be linear under the conditions used in these experiments with only slight evidence of saturation at very iong pulse intervals.
3.
120100..~ 80-
/~ernaL ~
4"~ 6 0 -
~ c o
I.O. 20"
~a----a u / _X_x__~,j@/
o ~ GasPhase x-- External (T0tat]
o / ~. -t,- ~.
6-- Exfernat (FirmPeak]
'
014 ' 0;6 ' o~ ' 1:0 ' Laser Shot Intervat [sec]
0'.2'
~',2 '
1'.~ '
1;6 1,6
Fig. 3. Dependence of the gas phase ionization, internal surface ionization and external surface ionization mass spectral signals on laser pulse repetition rate. For the external case, the total area under all peaks and that under just the first peak have been plotted separately. The intense peak displayed in fig. 1d appears at the same time as the dotted line of fig. 2, within experimental error. It is certainly expected that when U V light pulses pass through a purely gas phase sample, the ion yield per laser pulse should be independent o f the pulse repetition rate as long as the laser pulses are exactly reproducible. This is because gas phase molecules are completely mobile and any region that is depleted by ionization during one pulse can easily be replenished with molecules in less than a millisecond. This expected behavior is confirmed by the gas phase ionization data displayed in fig. 3 which indicates that
15 May 1989
Discussion
and
conclusion
In the "external" ionization mode the locations of the mass spectral peaks and the lack o f any dependence of these peaks on laser pulse repetition rate implies that gas phase, rather than surface adsorbed molecules, are being ionized. Using a simple computer program, it is possible to convert the observed multiple peak flight time distribution to a distribution o f ion formation positions. This leads to the position scale displayed in fig. 2. At an angle of incidence 0 = 0.28 °'the first antinode of the electric field is centered at about 14 pm above the metal surface. At 0 = 0 . 6 2 ° it is approximately 6 ~tm above the metal. At the surface itself, the light intensity vanishes. The fact that these spacings are much larger than the wavelength o f our UV light has been explained in an earlier paper [ 5 ]. When a laser beam passing through an ionizable gas irradiates a metal surface, gas phase ionization is apparently unavoidable. On the other hand, desorption/ionization experiments can be performed on nonvolatile molecules or those trapped on a cold surface or chemically bound to one, as long as the surface absorbs enough light. When UV laser radiation is incident on the metal film from the prism side, the ionization signal increases linearly with the time between laser pulses. Since all other mass spectrometer conditions and laser parameters were held constant, the increasing ion yield must result from an increasing density o f adsorbed molecules as the laser pulse repetition rate 195
Volume 71, number 3,4
OPTICS COMMUNICATIONS
decreases. As is evident from figs. 1b and 1d, the single mass peak obtained by internal reflection ionization appears at the dotted line position in fig. 2, where the surface ion peak is supposed to be. Although surface adsorption of the sample molecules is clearly implicated by this experiment, it is still not clear whether the neutral aniline molecules are initially desorbed and ionized very close to the surface (within the range o f the evanescent light wave) or ionized at the surface and then desorbed. A wavelength dependence experiment designed to probe this previously led to an inconclusive result in the case of aniline [4]. A more detailed study o f the repetition rate dependence in which the adsorption/desorption process is quantitatively interpreted and saturation effects are observed will be presented in a forthcoming publication [9 ]. The initial velocity distribution of the ions produced at the surface does not considerably broaden the TOF mass spectral peak obtained in the internal ionization mode, as is apparent from fig. l d. This fact has already been exploited in the development of an unusually high resolution time o f flight mass spectrometer [ 10]. While the slight temporal shift between the single, internal ionization mass spectral peak o f fig. 1d and the first of the multiple peaks o f fig. 1b results mainly from the different ionizing positions in the two cases, a small contribution to the shift arises from the nonzero initial velocities o f the neutral molecules a n d / o r ions desorbed from the surface. This contribution can be much more easily measured and studied when lower drawout electric fields are applied. We have done this in a series o f experiments and have extracted the initial velocity
196
15 May 1989
distribution o f the ions generated in the internal ionization mode. The results will be reported shortly [11].
Acknowledgements
This work has been supported by the National Science Foundation, Environmental Protection Agency and the Research Corporation. James P. ReiUy is a Camille and Henry Dreyfus Teacher-Scholar.
References
[ 1] N.H. Tolk, M. Traum, J.C. Tully and T.E. Madey, eds., Desorption induced by electronic transitions (DIET I) (Springer-Verlag, Berlin 1983). [2] W. Brenig and D. Menzel, eds., Desorption induced by electronic transitions (DIET II) (Springer-Verlag, Berlin 1986). [3] V.S. Antonov, V.S. Letokhov and A.N. Shibanov, Appl. Phys. 25 (1981) 71. [4] J.R. Millard, M. Yang and J.P. Reilly, J. Phys. Chem. 91 (1987) 4323. [ 5 ] J.W. Chai and J.P. Reilly, Optics Comm. 49 (1984) 51. [6] M. Yang, J.R. Millard and J.P. Reilly, Optics Comm. 55 (1985) 41. [ 7 ] M. Born and E. Wolf, Principles of optics (Pergamon, New York, 1975). [8] D.C. Larson, in: Physics of thin films, Vol. 6, eds. M.H. Francombe and R.W. Hoffman (Academic Press, New York, 1971 ) 81. [9] J.R. Millard, M. Yang and J.P. Reilly, to be published. [ 10 ] M. Yang and J.P. Reilly, Int. J. Mass Spectrom. Ion Processes
75 (1987) 209. [11 ] M. Yang, J.R. Millard and J.P. Reilly, J. Phys. Chem., submitted.
OPTICS COMMUNICATIONS
15 May 1989
C O M P A R I S O N O F T W O M E T H O D S O F UV L A S E R - I N D U C E D SURFACE I O N I Z A T I O N Mo Y A N G and James P. REILLY Indiana University, Chemistry Department, Bloomington, IN 4 7405, USA
Received 29 December 1988
Two different methods of UV laser-induced desorption ionization are compared by measuring the dependence of each on the laser pulse repetition rate. At an aniline sample pressure of 2.3 × l0 -s torr, the ionization signal obtained with a prism internal reflection geometry increases linearly with the time between laser pulses. In contrast, the signal obtained with light incident on the surface from the gas phase side is independent of the laser repetition rate, as in conventional gas phase ionization experiments. These observations, in conjunction with measurements of the time of flight peak distribution, enable us to ascertain the locations where ionization takes place in the two experiments. UV laser-induced surface ionization with prism internal reflection is found to be appropriate for studying adsorption/desorption kinetics in experiments where gas phase molecules are continuously adsorbing and desorbing from a metal surface.
1. Introduction One approach to monitoring molecules adsorbed on a metal surface is to b o m b a r d the surface with particles or light, desorb the adsorbate, and then ionize the molecules that have been ejected into the gas phase [ 1,2 ]. The latter step can be performed with electrons or through a resonant gas phase laser ionization process. By using relatively low incident light intensities, temperatures increments and damage at the surface can be minimized [ 3 ]. In a few recent studies, we have been investigating the mechanism of ionization that takes place when low intensity U V laser pulses b o m b a r d thin metal films that have been coated on fused silica substrates and mounted in the source region o f a time o f flight mass spectrometer [ 4 ]. Volatile aromatic molecules that have been introduced into the gas phase above the metal film adsorb on it to some extent and are probed by the incident laser pulse. We previously demonstrated that when the laser beam is incident upon the metal film from the gas phase side at angles approaching grazing incidence, an interference pattern is generated above the surface whose periodicity is much larger than the light wavelength. Molecules present in the gas phase at positions corresponding to the antinodes o f the radiation's electric field can be excited
and ionized, leading to time of flight mass spectra that exhibit multiple peaks [ 5 ]. Although we have often observed these mass spectral patterns, it has not been established whether the first peak in a multiple peak distribution results from the ionization o f gas phase or surface-adsorbed molecules. F r o m a close examination o f the time o f flight distributions obtained in this "external" surface irradiation mode this will now be clearly elucidated. In order to avoid the multiple gas phase ionization peaks in our time o f flight mass spectra, we previously showed that a thin metal film can be irradiated by UV light pulses incident from the substrate side and the resulting evanescent wave has sufficient intensity to excite and ionize molecules adsorbed to the film [ 6 ]. In this case, gas phase ionization at significant distances above the surface is repressed since the radiation's interference pattern must be contained within the substrate, and the evanescent light wave does not project far into the gas phase [ 7 ]. Although in the one case o f N O adsorbed onto a gold film the wavelength dependence o f the process indicated that laser-induced desorption precedes gas phase ionization [4], this result may not be general. However, we have suggested that those molecules that are ionized in this "internal reflection" mode must be adsorbed on the surface at least at the onset o f the 193
Volume 71, number 3,4
OPTICS COMMUNICATIONS
light pulse. Since the number of molecules adsorbed on our metal film surface depends on the equilibrium between adsorption and desorption processes, and since incident laser light increases the rate of molecular desorption, we have investigated the dependence of the observed ion yield on laser pulse repetition rate. The rather striking results to be presented demonstrate that this is an effective method of distinguishing sample molecules adsorbed on a surface from those moving about in the gas phase above it.
2. Experimental The apparatus used to record the laser ionization mass spectra of aromatic molecules in either the "external" or "internal" modes as defined above has been described previously [4 ]. Briefly, a thin film of gold was coated onto the hypotenuse of a right angle prism that was mounted in the source region of a UHV TOF laser ionization mass spectrometer. The gold film's thickness was estimated by a resistance measurement to be about 0.5 to l nm [8]. 260 nm UV laser pulses generated by frequency doubling the output of a tunable dye laser irradiated the metal film from either the gas phase or fused silica side. The 1 microjoule light pulses were typically l to 2 nanoseconds in duration and were focussed to a diameter of about 0.1 mm at the metal surface. Since one of the principal experiments involved varying the laser repetition rate while keeping all other parameters constant, it was necessary to devise an arrangement that maintained a constant laser output energy and beam profile as its repetition rate changed by a factor of 30. This was accomplished by operating the laser at a constant repetition rate of 20 Hz and by employing a mechanical chopper that transmitted light pulses at selected intervals. With this arrangement repetition rates between 0.8 and 20 Hz were easily accessible. Aniline vapor at a pressure of 2.3 X l0 -8 torr was maintained in the gas phase above the metal surface. Laser-generated parent ions of mass 93 daltons were accelerated by a high ion drawout field (2000 volt across a 6.2 mm gap) toward a microchannel plate detector. The detector output was amplified, digitized and stored in an IBM/AT computer. Typical 194
15 May 1989
experiments involved signal averaging the results of 1000 light pulses. Fig. i displays the external and internal geometries that were used to record laser ionization mass spectra. Experimental data, in the form of a multiple peak mass spectrum, obtained with the former geometry, and a single sharp peak, obtained the the latter, are also displayed. (The small, second peak in the latter mass spectrum arises from re~z=94 ions present due to the natural abundance of ~ac.) In order to ascertain whether the first peak in fig. lb results from ionization of aniline at the gold film surface or at the first antinode of the light interference pattern above the surface we recorded multiple peak mass spectra at near-grazing angles of incidence, 0, equal to 0.28 and 0.62 °. The spectrometer fields and all other experimental parameters were held constant. The resulting mass spectra are displayed in fig. 2. It is clear that the first peaks in the two mass spectra do not occur at the same point in time, indicating that they do not arise from ionization of surface-adsorbed molecules. In fact, the Fresnel equations indicate that a node of the electric field should occur at the metal/gas boundary and the first antinode should be at a height above the surface equal to half of the periodicity of the interference pattern. This implies that an ion formed at the surface with zero initial kinetic energy should have a flight time given by the location of the dotted lines in figs. 2a and 2b. a) TOF MS
6
c) TOF MS
t
s'o
~6o
1~o
z6o
d l 6
s'o ~oo ~o 260 Channel Number [lch=2ns]
Fig. I. Comparison of two different UV laser-induced surface ionization methods. (a), (b) External reflection geometryand measured mass spectrum. (c), (d) Internal reflection geometry and measured mass spectrum.
Volume 71 number 3,4
OPTICS COMMUNICATIONS
0 I
20 i
i
40 n I
'/I//~ i
so
60 i
I
80 n
I
g
100 Jam I
I
: 0,2B*
40
1~0
b) i
' 50
0
= 0,62"
100
l~aO
Channet Number [lch=2ns]
Fig. 2. Laser ionization TOF mass spectra obtained in the external reflection geometry and displayed on an expanded time scale for two different anglesof incidence. (a) 0=0.28% (b) 0=0.62 °. The calculated position scale is indicated at the top.
the ion yield per laser shot is independent of the laser pulse repetition rate. Based on our interpretation o f the multiple peak mass spectra as described above, one would expect that their intensity would also be independent o f the laser shot interval. In fig. 3 we demonstrate that both the total area o f all o f the multiple peaks and the area o f the first o f the multiple peaks are independent o f laser shot interval. In contrast, the area of the single sharp peak generated in the internal ionization geometry is seen to be highly dependent on the repetition rate of the laser. The dependence appears to be linear under the conditions used in these experiments with only slight evidence of saturation at very iong pulse intervals.
3.
120100..~ 80-
/~ernaL ~
4"~ 6 0 -
~ c o
I.O. 20"
~a----a u / _X_x__~,j@/
o ~ GasPhase x-- External (T0tat]
o / ~. -t,- ~.
6-- Exfernat (FirmPeak]
'
014 ' 0;6 ' o~ ' 1:0 ' Laser Shot Intervat [sec]
0'.2'
~',2 '
1'.~ '
1;6 1,6
Fig. 3. Dependence of the gas phase ionization, internal surface ionization and external surface ionization mass spectral signals on laser pulse repetition rate. For the external case, the total area under all peaks and that under just the first peak have been plotted separately. The intense peak displayed in fig. 1d appears at the same time as the dotted line of fig. 2, within experimental error. It is certainly expected that when U V light pulses pass through a purely gas phase sample, the ion yield per laser pulse should be independent o f the pulse repetition rate as long as the laser pulses are exactly reproducible. This is because gas phase molecules are completely mobile and any region that is depleted by ionization during one pulse can easily be replenished with molecules in less than a millisecond. This expected behavior is confirmed by the gas phase ionization data displayed in fig. 3 which indicates that
15 May 1989
Discussion
and
conclusion
In the "external" ionization mode the locations of the mass spectral peaks and the lack o f any dependence of these peaks on laser pulse repetition rate implies that gas phase, rather than surface adsorbed molecules, are being ionized. Using a simple computer program, it is possible to convert the observed multiple peak flight time distribution to a distribution o f ion formation positions. This leads to the position scale displayed in fig. 2. At an angle of incidence 0 = 0.28 °'the first antinode of the electric field is centered at about 14 pm above the metal surface. At 0 = 0 . 6 2 ° it is approximately 6 ~tm above the metal. At the surface itself, the light intensity vanishes. The fact that these spacings are much larger than the wavelength o f our UV light has been explained in an earlier paper [ 5 ]. When a laser beam passing through an ionizable gas irradiates a metal surface, gas phase ionization is apparently unavoidable. On the other hand, desorption/ionization experiments can be performed on nonvolatile molecules or those trapped on a cold surface or chemically bound to one, as long as the surface absorbs enough light. When UV laser radiation is incident on the metal film from the prism side, the ionization signal increases linearly with the time between laser pulses. Since all other mass spectrometer conditions and laser parameters were held constant, the increasing ion yield must result from an increasing density o f adsorbed molecules as the laser pulse repetition rate 195
Volume 71, number 3,4
OPTICS COMMUNICATIONS
decreases. As is evident from figs. 1b and 1d, the single mass peak obtained by internal reflection ionization appears at the dotted line position in fig. 2, where the surface ion peak is supposed to be. Although surface adsorption of the sample molecules is clearly implicated by this experiment, it is still not clear whether the neutral aniline molecules are initially desorbed and ionized very close to the surface (within the range o f the evanescent light wave) or ionized at the surface and then desorbed. A wavelength dependence experiment designed to probe this previously led to an inconclusive result in the case of aniline [4]. A more detailed study o f the repetition rate dependence in which the adsorption/desorption process is quantitatively interpreted and saturation effects are observed will be presented in a forthcoming publication [9 ]. The initial velocity distribution of the ions produced at the surface does not considerably broaden the TOF mass spectral peak obtained in the internal ionization mode, as is apparent from fig. l d. This fact has already been exploited in the development of an unusually high resolution time o f flight mass spectrometer [ 10]. While the slight temporal shift between the single, internal ionization mass spectral peak o f fig. 1d and the first of the multiple peaks o f fig. 1b results mainly from the different ionizing positions in the two cases, a small contribution to the shift arises from the nonzero initial velocities o f the neutral molecules a n d / o r ions desorbed from the surface. This contribution can be much more easily measured and studied when lower drawout electric fields are applied. We have done this in a series o f experiments and have extracted the initial velocity
196
15 May 1989
distribution o f the ions generated in the internal ionization mode. The results will be reported shortly [11].
Acknowledgements
This work has been supported by the National Science Foundation, Environmental Protection Agency and the Research Corporation. James P. ReiUy is a Camille and Henry Dreyfus Teacher-Scholar.
References
[ 1] N.H. Tolk, M. Traum, J.C. Tully and T.E. Madey, eds., Desorption induced by electronic transitions (DIET I) (Springer-Verlag, Berlin 1983). [2] W. Brenig and D. Menzel, eds., Desorption induced by electronic transitions (DIET II) (Springer-Verlag, Berlin 1986). [3] V.S. Antonov, V.S. Letokhov and A.N. Shibanov, Appl. Phys. 25 (1981) 71. [4] J.R. Millard, M. Yang and J.P. Reilly, J. Phys. Chem. 91 (1987) 4323. [ 5 ] J.W. Chai and J.P. Reilly, Optics Comm. 49 (1984) 51. [6] M. Yang, J.R. Millard and J.P. Reilly, Optics Comm. 55 (1985) 41. [ 7 ] M. Born and E. Wolf, Principles of optics (Pergamon, New York, 1975). [8] D.C. Larson, in: Physics of thin films, Vol. 6, eds. M.H. Francombe and R.W. Hoffman (Academic Press, New York, 1971 ) 81. [9] J.R. Millard, M. Yang and J.P. Reilly, to be published. [ 10 ] M. Yang and J.P. Reilly, Int. J. Mass Spectrom. Ion Processes
75 (1987) 209. [11 ] M. Yang, J.R. Millard and J.P. Reilly, J. Phys. Chem., submitted.