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Laser multiphoton-ionization detection of molecules adsorbed on a surface
.oFMoI;ECUI;ES~ADS~RBED-~~.~-SURFACE . .,:_. . ..I*1... S.E. EGOROV;:V.S. ii3oKH0vi~1d A.N.SHIBANOV Inhue
. _._ -.
oi SpcZroscopy, USSR A&demj- of S&nces. Troitzk. Mhrcow region, 142092 US.$R
:
-.
.:
., -,
.--
Received 9 May 1983;.in fmal form 20 December 1983
Ion formation from &olecules adsorbed on a’surface under the action of UV laser pulses has been studied. A method for increasing .significantly the detection sensitivity of small amounts of molecules in a pho:oionixation mass spectrometer is proposed. The increase is due to accumulation of molecules on a cooled substrate and subsequent pulseddesorption and photoionization above the surface. An experimental increase in the detectivity of naphthalene of 2 x lo3 has been demonstrated.. _,
1. Introduction The application of various schemes of selective stepwise laser-induced photoionization has been proposed recently for detecting small amounts of atoms and molecules [1,2]. The possibility of creating a laser selective molecular detector by combining selective two;step photoionization of molecules and mass-spectroscopic analysis .of the resultant photofragments’ was discussed in ref. 131. For the detection and identification of.low concentrations of polyatomic molecules it is important to ionize a considerable part of them at moderate fragmentation. This becomes possible when laser radiation with enough power resonant tc an electronic transition is used. The first experiments on laser-induced stepwise photoionization of molecules, via an excited electronic state were. described in: ref. [4], and then many papers on multiphoton and multistep ionization of polyatomic molecules -were published (see ref. [S]). Antonov etal: [6] ::detected single molecules of naphthaline in aphotoionization volume, demonstrating for-the-first:~.~-the_ultimate.detec.. tion sensitivity.of.~thistechnique.~ ’ The present -_paper-:de&bes a t method .&I&h provides. - an essential-increase:-. in- the” detection _~ _-
sensitivity of low molecular concentrations. The idea is that molecules are accumulated on a cooled surface due to -adsorption and then pulsed laser desorption and stepwise laser photoionization of these molecules above. the surface. are performed. The detection sensitivity of naphthaiene %nd anthracene molecules has been-increased by more than lo?- times. Applications of the method are discussed.
2. Experimental
and results
_ In .-our experiment we used an arrangement similar-to the one described in ref. [7] (fig. .l). Its basic element-was a time-of-flight mass spectrometer; Finely-dispersed crystalline powder of adenine or a polycrystalline- film of rhodamine 6G placed on a copper substrate we& used as adsorbates. The sample, was placed -on the repellent electrode ,of. the: ionization .-chamber.: of the.. time-ofiflightmass spectrometer;.~k.laser-pulses_passe’4~ through the quartz side kindow of the mass spectrometer and; after reflection by-the mirror in the_ioniz&ion .chamber, -fell on. the Lsample at. 4Y _.to ihe I mass _- I spectrometer +xis..
0301-0104/84/$03.00 0. Elsevier$cience Publishers BY. (North-Holland PhysicsPublishirig .Division) -
. .
’
350
S. E. Egomu er al. /
Fig. 1. Experimental
Mulriphoron -ionization detection of molerules
arrangement.
In the experiment the pulses of a KrF laser (X = 249 nm. Q = 20 ns) and the second harmonic of a Nd : YAG laser (A = 531 nm, r,, = 10 ns) were used. The pulse from a photodiode irradiated by a Nd: YAG laser pulse with a delay time of 2 ps triggered the KrF laser. The first experiments were performed with a
l
T=200
finely-dispersed crystalline powder of adenine. In ref. [7] it was found that irradiation of adenine with UV laser pulses leads to the formation of adenine molecular ions. It has been found in the present work that, if the mass-spectrometer volume contains anthracene vapour, one can observe both adenine and anthracene ions in the mass spectrum. As the volume was irradiated, only anthracene ions were observed above the surface, but the signal at + = 6 mJ/cm’ was two or three times weaker. Cooling of the substrate with the sample to T = 200 K brought about a considerable reduction of the vapour pressure of anthracene, and a corresponding decrease of the gas-phase anthracene ion signal. But the signal from the surface increased significantly. This can be explained as follows. The anthracene ions are formed from the anthracene molecules adsorbed on the adenine surface when adenine crystals are irradiated. When the laser fluence equaled 10 mJ/cm’, the signal from the surface was more than 200 times stronger than the signal from the gas phase. Fig. 2A shows
‘K
?
+T=300-K
64-
FLUENCE,
211llllll 4
J/cm’
Fig. 2. Dependence of the yield lkence when (A) adenine powder irradiated.
6
. 1,, 11 8 1()-3 6 8 IO-2 FLUENCE.
of anthracene ions on laser or (B) rhodamine 6G film is
I
.
2
J/cm2
Fig. 3. Dependence of the yield of naphthalene ion on fluence of a second laser pulse. Adsorbate is a film of rhodamine 6G.
S. E. Egorov et al_ / Multiphoton -ioni+ion
the ion signal .-for anthracene -adsorbed on the adenine surface as a function of the laser fluence +_ At T F 200 K the dependence can be approximated as U cc@ and,. at -T = 300 K,- it is close to the dependence of the ionization yield of anthracene vapour on laser fluence. in the case of anthracene adsorption on the surface of rhodamine 6G (fig. 2B) the corresponding dependences vary from Ua q9 at T = 200 K to Ua +a.5at T = 300 K. In this case the ratio of the signal from the surface to the signal from the gas phase at T = 200 K and + = 16 mJ/cm*, exceeded 103. Experiments with two laser pulses were performed. A polycrystalline film of rhodamine 6G (T = 200 K) with naphthalene molecules adsorbed on it was irradiated first by a pulse with X, = 531 nm and C#Q = 4.3 mJ/cm*. No ions were formed under the action of this pulse. Then with a delay time Q = 3 ~.LSan UV pulse with X2 = 249 nm and +? = 1 mJ/cm’ was applied. A strong naphthalene ion signal was observed in this case, independent of the fluence of the first pulse which varied from 4 to 6 mJ/cm* and quadratically dependent on the fluence of the second-pulse (fig. 3, left-hand side). For comparison, fig.3, right-hand side shows the corresponding dependence for the case when the surface is irradiated by one KrF laser pulse. The irradiation of the volume above the sample surface with KrF laser pulses gave rise to naphthalene ion formation but the signal was 2 x 10” times weaker than the signal formed by irradiating the sample surface with two pulses. In the above experiments the time intervals between pulses (or pairs of pulses in the case of the two-laser experiment) were a few seconds. Measurements with increasingly longer time intervals (up to a few minutes) were made, but the signal did not change. This means that the steady state was reached between the shots.
3. Discussion Antonov et al. [7] observed molecular ions when molecular crystals were irradiated with UV laser pulses of moderate intensity. Subsequent experiments made it possible to reveal the following
351
detection of molecules
mechanism of this effect-(see refs.,[8,9] land refer: ences therein). Laser irradiation induces moderate heating of the -sample surface .a+ evaporation of neutral molecules which are. photoionized above the surface via an intermediate electronicstate by the same UV laser pulse. When adsorbed molecules are present on the sample surface, the mass spectrum shows molecular ions of both the adsorbate and the adsorbent. The dependenccs observed (fig. 2) can be explained within the framework of this mechanism. For anthracene on the surface of adenine at low temperatures of the sample, the number of anthracene ions is determined by the number of molecules desorbed during a pulse and by the ionization efficiency. The desorption rate is strongly dependent on the surface temperature and hence on the laser fluence. At T = 300 K, all anthracene molecules desorb from the adenine surface at the beginning of a laser pulse, so that the ion yield is determined mainly by the ionization efficiency in the gas phase. For anthracene on the surface of rhodamine 6G the adsorption energy is higher, so the corresponding dependence is steeper and, even with an initial sample temperature of T = 300 K, there is no complete desorption at the beginning of the laser pulse. The dependence.of the ion yield on the laser fluence is still far from quadratic.
4. Enhancement
of detection
sensitivity
The experimental results presented show that the method of molecular adsorption on a surface and pulsed desorption can be applied to increase the detection sensitivity of molecules in a photoionization mass spectrometer. The increase in sensitivity is due to the fact that the number- of
molecules on the surface under irradiation under certain conditions can substantially exceed the quantity of molecules in the irradiated volume. The change in the molecular concentration on the surface is determined by the difference of molecular fluxes onto the surface and from the surface dnsufi/dt
= n v&&4
- nsurf/r7
where ns,,r and n vap are the molecular
0) concentra-
352
SE.
Egorm
et al. /
Multiphoton-ionization
tions on the surface and in the vapour respectively; u = (Sk’T/mn)‘/’ IS - the mean molecular velocity in the vapour, T is the residence time of the molecules adsorbed on the surface. The residence time T is related to the adsorption energy E and the surface temperature T by the Arrhenius formula “Y-k f0 exp( E/kT).
(2)
where t0 = 10-‘z-10-‘4 s is the characteristic vibrational period of molecules on the surface. During the time t >, T the equilibrium concentration nsurl = n vopUT/~ is reached. A maximum ratio of ion signals from the surface and from the vapour can be attained at full desorption of the molecules from the surface. This value is determined by the ratio between the number of molecules on the surface under irradiation S and that in the photoionization volume V
If S is the laser-beam length of the irradiated this ratio equalsu,,,r/L/,.,,
=
VT/41.
cross-section and I is the volume above the surface,
(4)
is the ratio of the distance travelled by a molecule during the time T to the characteristic size I of the photoionization volume. Thus, the method discussed is equivalent to an increase of the length of the ionization volume by a factor of 07/41. With u = 10’ cm/s. T = 1 s. I = 1 cm. Us,,f/U,.ap = 10’. To obtain a maximum efficiency of this method with one pulse in use it is necessary that full desorption of molecules should occur at the beginning of a pulse. In other words, when a small fraction of the pulse energy is absorbed. the surface must be heated to a temperature at which the residence time of molecules on the surface is shorter than the pulse duration. But in this case absorption of the entire pulse energy and an additional substantial increase in temperature can cause undesirable effects related to intense evaporation of the adsorbate. Besides, the use of one pulse makes it impossible to perform independent optimization
detection of molecules
--
of the desorption
and photoionization. processes; Both disadvantages can be’ eliminated by using two pulses with a delay between them. In this case a full desorption of the molecules should occur before the arrival of the second ionizing pulse, and the requirement on sample heating with the first pulse is reduced. For example in the experiment described above with naphthalene adsorbed on a film of rhodamine 6G, the maximum ratio Usurf/Uvapwas equal to 17 at C#I = 16 mJ/cm’ while one KrF pulse was used. A further increase of laser fluence resulted in distinct thermal damage of the rhodamine 6G film. At the same time, the use of two pulses (+r = 4.3 mJ/cm’, X, =531 nm and C& = 1 mJ/cm’. X, = 249 nm) provided an ultimate value = 2 X 10’ at the given initial temperaq!r,rr/ I/vap ture of the sample. At full desorption of molecules with the first pulse it is easy to find the residence time T of the molecules adsorbed on the surface from eq. (3) and estimate the adsorption energy from relation (2) by measuring the ratio U,,,,/U,,,,. For example from the experimental value of 2 x lo3 obtained at T = 200 K for naphthalene molecules adsorbed on a rhodamine 6G surface we obtain T = 0.18 s and E = 0.5 eV_
VT//
5. Conclusion The experimental results presented in this paper show that by applying the method of molecular adsorption on a surface with subsequent pulsed desorption and photoionization, it is possible to increase the sensitivity of photoionization detection of molecules by several orders of magnitude. The highest sensitivity can be attained when two laser pulses are used. The first pulse carriers out full desorption and the second one performs the ionization_ By choosing proper parameters of the second pulse one can realize highly efficient selective ionization of molecules. The lasers used in this case may have a low repetition rate. It is not difficult to imagine a version of such a detector in which the molecules desorbed by pulses fall within a pulsed supersonic jet for cooling and subsequent selective photoionization detection.
S.E. Egorou et al. / Multiphoton - ionization detection of molecules
Acknowledgement The authors wish to express their gratitude to Drs. Yu.A. Matveets and S.V. Chekalin who provided them with a Nd: YAG laser to perform the two-pulse experiment.
References [l] V.S. Letokhov. in: Frontiers in laser spectroscopy, Vol. 2. eds. R. Balian, S. Haroche and S. Liberman (North-Holland, Amsterdam. 1977) p. 771. [2] VS. Letokhov. in: Springer series in optical sciences, Vol. 3. Tunable lasers and applications, eds. A. Mooradian. T. Jarger and P. Stokseth (Springer, Berlin. 1976) p. 122.
353
i31 R.V. Ambartzumian
and VS. Letokhov, Appl. Opt. 11 (1972) 354. V.S. Antonov, LN. Knyazev and V.S. r41 S.V. Andreev, Letokhov, Chem. Phys. Letters 45 (1977) 166. PI P.M. Johnson. Accounts Chem. Res. 13 (1980) 20; V.S. Antonov and V.S. Letokhov, Appl. Phys. 24 41981) 89. VS. Letokhov and A.N. Shibanov. Opt. PI V.S. Antonov.
Commun. 38 (1981) 182. V.S. Letokhov and A.N. Shibanov, Pis’ma t71 VS. Antonov, Zh. Experim. i Tear. Fiz. 31 (1980) 471 (in Russian); V.S. Antonov. VS. Letokhov and A.N. Shivanov, Appl. Phys. 25 (1981) 71. 181SE. Egorov. V.S. Letokhov and A.N. Shibanov, in: Springer series in chemical physics. Surface studies with lasers, eds.
PI
F.R. Aussenegg, A. Leitner and M.E. Lippitsch (Springer, Berlin. 1983). SE. Egorov. VS. Letokhov and A.N. Shibanov, Kvantovaya Elektron, to be published (in Russian).
. _._ -.
oi SpcZroscopy, USSR A&demj- of S&nces. Troitzk. Mhrcow region, 142092 US.$R
:
-.
.:
., -,
.--
Received 9 May 1983;.in fmal form 20 December 1983
Ion formation from &olecules adsorbed on a’surface under the action of UV laser pulses has been studied. A method for increasing .significantly the detection sensitivity of small amounts of molecules in a pho:oionixation mass spectrometer is proposed. The increase is due to accumulation of molecules on a cooled substrate and subsequent pulseddesorption and photoionization above the surface. An experimental increase in the detectivity of naphthalene of 2 x lo3 has been demonstrated.. _,
1. Introduction The application of various schemes of selective stepwise laser-induced photoionization has been proposed recently for detecting small amounts of atoms and molecules [1,2]. The possibility of creating a laser selective molecular detector by combining selective two;step photoionization of molecules and mass-spectroscopic analysis .of the resultant photofragments’ was discussed in ref. 131. For the detection and identification of.low concentrations of polyatomic molecules it is important to ionize a considerable part of them at moderate fragmentation. This becomes possible when laser radiation with enough power resonant tc an electronic transition is used. The first experiments on laser-induced stepwise photoionization of molecules, via an excited electronic state were. described in: ref. [4], and then many papers on multiphoton and multistep ionization of polyatomic molecules -were published (see ref. [S]). Antonov etal: [6] ::detected single molecules of naphthaline in aphotoionization volume, demonstrating for-the-first:~.~-the_ultimate.detec.. tion sensitivity.of.~thistechnique.~ ’ The present -_paper-:de&bes a t method .&I&h provides. - an essential-increase:-. in- the” detection _~ _-
sensitivity of low molecular concentrations. The idea is that molecules are accumulated on a cooled surface due to -adsorption and then pulsed laser desorption and stepwise laser photoionization of these molecules above. the surface. are performed. The detection sensitivity of naphthaiene %nd anthracene molecules has been-increased by more than lo?- times. Applications of the method are discussed.
2. Experimental
and results
_ In .-our experiment we used an arrangement similar-to the one described in ref. [7] (fig. .l). Its basic element-was a time-of-flight mass spectrometer; Finely-dispersed crystalline powder of adenine or a polycrystalline- film of rhodamine 6G placed on a copper substrate we& used as adsorbates. The sample, was placed -on the repellent electrode ,of. the: ionization .-chamber.: of the.. time-ofiflightmass spectrometer;.~k.laser-pulses_passe’4~ through the quartz side kindow of the mass spectrometer and; after reflection by-the mirror in the_ioniz&ion .chamber, -fell on. the Lsample at. 4Y _.to ihe I mass _- I spectrometer +xis..
0301-0104/84/$03.00 0. Elsevier$cience Publishers BY. (North-Holland PhysicsPublishirig .Division) -
. .
’
350
S. E. Egomu er al. /
Fig. 1. Experimental
Mulriphoron -ionization detection of molerules
arrangement.
In the experiment the pulses of a KrF laser (X = 249 nm. Q = 20 ns) and the second harmonic of a Nd : YAG laser (A = 531 nm, r,, = 10 ns) were used. The pulse from a photodiode irradiated by a Nd: YAG laser pulse with a delay time of 2 ps triggered the KrF laser. The first experiments were performed with a
l
T=200
finely-dispersed crystalline powder of adenine. In ref. [7] it was found that irradiation of adenine with UV laser pulses leads to the formation of adenine molecular ions. It has been found in the present work that, if the mass-spectrometer volume contains anthracene vapour, one can observe both adenine and anthracene ions in the mass spectrum. As the volume was irradiated, only anthracene ions were observed above the surface, but the signal at + = 6 mJ/cm’ was two or three times weaker. Cooling of the substrate with the sample to T = 200 K brought about a considerable reduction of the vapour pressure of anthracene, and a corresponding decrease of the gas-phase anthracene ion signal. But the signal from the surface increased significantly. This can be explained as follows. The anthracene ions are formed from the anthracene molecules adsorbed on the adenine surface when adenine crystals are irradiated. When the laser fluence equaled 10 mJ/cm’, the signal from the surface was more than 200 times stronger than the signal from the gas phase. Fig. 2A shows
‘K
?
+T=300-K
64-
FLUENCE,
211llllll 4
J/cm’
Fig. 2. Dependence of the yield lkence when (A) adenine powder irradiated.
6
. 1,, 11 8 1()-3 6 8 IO-2 FLUENCE.
of anthracene ions on laser or (B) rhodamine 6G film is
I
.
2
J/cm2
Fig. 3. Dependence of the yield of naphthalene ion on fluence of a second laser pulse. Adsorbate is a film of rhodamine 6G.
S. E. Egorov et al_ / Multiphoton -ioni+ion
the ion signal .-for anthracene -adsorbed on the adenine surface as a function of the laser fluence +_ At T F 200 K the dependence can be approximated as U cc@ and,. at -T = 300 K,- it is close to the dependence of the ionization yield of anthracene vapour on laser fluence. in the case of anthracene adsorption on the surface of rhodamine 6G (fig. 2B) the corresponding dependences vary from Ua q9 at T = 200 K to Ua +a.5at T = 300 K. In this case the ratio of the signal from the surface to the signal from the gas phase at T = 200 K and + = 16 mJ/cm*, exceeded 103. Experiments with two laser pulses were performed. A polycrystalline film of rhodamine 6G (T = 200 K) with naphthalene molecules adsorbed on it was irradiated first by a pulse with X, = 531 nm and C#Q = 4.3 mJ/cm*. No ions were formed under the action of this pulse. Then with a delay time Q = 3 ~.LSan UV pulse with X2 = 249 nm and +? = 1 mJ/cm’ was applied. A strong naphthalene ion signal was observed in this case, independent of the fluence of the first pulse which varied from 4 to 6 mJ/cm* and quadratically dependent on the fluence of the second-pulse (fig. 3, left-hand side). For comparison, fig.3, right-hand side shows the corresponding dependence for the case when the surface is irradiated by one KrF laser pulse. The irradiation of the volume above the sample surface with KrF laser pulses gave rise to naphthalene ion formation but the signal was 2 x 10” times weaker than the signal formed by irradiating the sample surface with two pulses. In the above experiments the time intervals between pulses (or pairs of pulses in the case of the two-laser experiment) were a few seconds. Measurements with increasingly longer time intervals (up to a few minutes) were made, but the signal did not change. This means that the steady state was reached between the shots.
3. Discussion Antonov et al. [7] observed molecular ions when molecular crystals were irradiated with UV laser pulses of moderate intensity. Subsequent experiments made it possible to reveal the following
351
detection of molecules
mechanism of this effect-(see refs.,[8,9] land refer: ences therein). Laser irradiation induces moderate heating of the -sample surface .a+ evaporation of neutral molecules which are. photoionized above the surface via an intermediate electronicstate by the same UV laser pulse. When adsorbed molecules are present on the sample surface, the mass spectrum shows molecular ions of both the adsorbate and the adsorbent. The dependenccs observed (fig. 2) can be explained within the framework of this mechanism. For anthracene on the surface of adenine at low temperatures of the sample, the number of anthracene ions is determined by the number of molecules desorbed during a pulse and by the ionization efficiency. The desorption rate is strongly dependent on the surface temperature and hence on the laser fluence. At T = 300 K, all anthracene molecules desorb from the adenine surface at the beginning of a laser pulse, so that the ion yield is determined mainly by the ionization efficiency in the gas phase. For anthracene on the surface of rhodamine 6G the adsorption energy is higher, so the corresponding dependence is steeper and, even with an initial sample temperature of T = 300 K, there is no complete desorption at the beginning of the laser pulse. The dependence.of the ion yield on the laser fluence is still far from quadratic.
4. Enhancement
of detection
sensitivity
The experimental results presented show that the method of molecular adsorption on a surface and pulsed desorption can be applied to increase the detection sensitivity of molecules in a photoionization mass spectrometer. The increase in sensitivity is due to the fact that the number- of
molecules on the surface under irradiation under certain conditions can substantially exceed the quantity of molecules in the irradiated volume. The change in the molecular concentration on the surface is determined by the difference of molecular fluxes onto the surface and from the surface dnsufi/dt
= n v&&4
- nsurf/r7
where ns,,r and n vap are the molecular
0) concentra-
352
SE.
Egorm
et al. /
Multiphoton-ionization
tions on the surface and in the vapour respectively; u = (Sk’T/mn)‘/’ IS - the mean molecular velocity in the vapour, T is the residence time of the molecules adsorbed on the surface. The residence time T is related to the adsorption energy E and the surface temperature T by the Arrhenius formula “Y-k f0 exp( E/kT).
(2)
where t0 = 10-‘z-10-‘4 s is the characteristic vibrational period of molecules on the surface. During the time t >, T the equilibrium concentration nsurl = n vopUT/~ is reached. A maximum ratio of ion signals from the surface and from the vapour can be attained at full desorption of the molecules from the surface. This value is determined by the ratio between the number of molecules on the surface under irradiation S and that in the photoionization volume V
If S is the laser-beam length of the irradiated this ratio equalsu,,,r/L/,.,,
=
VT/41.
cross-section and I is the volume above the surface,
(4)
is the ratio of the distance travelled by a molecule during the time T to the characteristic size I of the photoionization volume. Thus, the method discussed is equivalent to an increase of the length of the ionization volume by a factor of 07/41. With u = 10’ cm/s. T = 1 s. I = 1 cm. Us,,f/U,.ap = 10’. To obtain a maximum efficiency of this method with one pulse in use it is necessary that full desorption of molecules should occur at the beginning of a pulse. In other words, when a small fraction of the pulse energy is absorbed. the surface must be heated to a temperature at which the residence time of molecules on the surface is shorter than the pulse duration. But in this case absorption of the entire pulse energy and an additional substantial increase in temperature can cause undesirable effects related to intense evaporation of the adsorbate. Besides, the use of one pulse makes it impossible to perform independent optimization
detection of molecules
--
of the desorption
and photoionization. processes; Both disadvantages can be’ eliminated by using two pulses with a delay between them. In this case a full desorption of the molecules should occur before the arrival of the second ionizing pulse, and the requirement on sample heating with the first pulse is reduced. For example in the experiment described above with naphthalene adsorbed on a film of rhodamine 6G, the maximum ratio Usurf/Uvapwas equal to 17 at C#I = 16 mJ/cm’ while one KrF pulse was used. A further increase of laser fluence resulted in distinct thermal damage of the rhodamine 6G film. At the same time, the use of two pulses (+r = 4.3 mJ/cm’, X, =531 nm and C& = 1 mJ/cm’. X, = 249 nm) provided an ultimate value = 2 X 10’ at the given initial temperaq!r,rr/ I/vap ture of the sample. At full desorption of molecules with the first pulse it is easy to find the residence time T of the molecules adsorbed on the surface from eq. (3) and estimate the adsorption energy from relation (2) by measuring the ratio U,,,,/U,,,,. For example from the experimental value of 2 x lo3 obtained at T = 200 K for naphthalene molecules adsorbed on a rhodamine 6G surface we obtain T = 0.18 s and E = 0.5 eV_
VT//
5. Conclusion The experimental results presented in this paper show that by applying the method of molecular adsorption on a surface with subsequent pulsed desorption and photoionization, it is possible to increase the sensitivity of photoionization detection of molecules by several orders of magnitude. The highest sensitivity can be attained when two laser pulses are used. The first pulse carriers out full desorption and the second one performs the ionization_ By choosing proper parameters of the second pulse one can realize highly efficient selective ionization of molecules. The lasers used in this case may have a low repetition rate. It is not difficult to imagine a version of such a detector in which the molecules desorbed by pulses fall within a pulsed supersonic jet for cooling and subsequent selective photoionization detection.
S.E. Egorou et al. / Multiphoton - ionization detection of molecules
Acknowledgement The authors wish to express their gratitude to Drs. Yu.A. Matveets and S.V. Chekalin who provided them with a Nd: YAG laser to perform the two-pulse experiment.
References [l] V.S. Letokhov. in: Frontiers in laser spectroscopy, Vol. 2. eds. R. Balian, S. Haroche and S. Liberman (North-Holland, Amsterdam. 1977) p. 771. [2] VS. Letokhov. in: Springer series in optical sciences, Vol. 3. Tunable lasers and applications, eds. A. Mooradian. T. Jarger and P. Stokseth (Springer, Berlin. 1976) p. 122.
353
i31 R.V. Ambartzumian
and VS. Letokhov, Appl. Opt. 11 (1972) 354. V.S. Antonov, LN. Knyazev and V.S. r41 S.V. Andreev, Letokhov, Chem. Phys. Letters 45 (1977) 166. PI P.M. Johnson. Accounts Chem. Res. 13 (1980) 20; V.S. Antonov and V.S. Letokhov, Appl. Phys. 24 41981) 89. VS. Letokhov and A.N. Shibanov. Opt. PI V.S. Antonov.
Commun. 38 (1981) 182. V.S. Letokhov and A.N. Shibanov, Pis’ma t71 VS. Antonov, Zh. Experim. i Tear. Fiz. 31 (1980) 471 (in Russian); V.S. Antonov. VS. Letokhov and A.N. Shivanov, Appl. Phys. 25 (1981) 71. 181SE. Egorov. V.S. Letokhov and A.N. Shibanov, in: Springer series in chemical physics. Surface studies with lasers, eds.
PI
F.R. Aussenegg, A. Leitner and M.E. Lippitsch (Springer, Berlin. 1983). SE. Egorov. VS. Letokhov and A.N. Shibanov, Kvantovaya Elektron, to be published (in Russian).