- Email: [email protected]
Femtosecond (2+1′) pump-probe mass spectroscopy on the Rydberg states of ammonia
22 April 2002
Chemical Physics Letters 356 (2002) 227–232 www.elsevier.com/locate/cplett
Femtosecond (2 þ 10) pump-probe mass spectroscopy on the Rydberg states of ammonia Shu-Hui Yin, Hong-Ping Liu, Jian-Yang Zhang, Bo Jiang, Da-Li Xu, Li Wang *, Guo-He Sha, Nan-Quan Lou State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China Received 28 September 2001; in final form 13 February 2002
Abstract The Rydberg state decay dynamics of ammonia is investigated in real-time by (2 þ 10 ) femtosecond pump-probe ~ 0 1 A0 state is determined as 936 92 fs for the first time. The ionization detection. The lifetime of ammonia in the E 1 ~ 0 1 A0 state has been discussed in this report. Concurrently, internal conversion (IC) pre-dissociation mechanism of the E 1 1 00 0 ~ the A A2 state ðm2 ¼ 4Þ lifetime is estimated as 51 4 fs, which is identical with the previous reports. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Fast intra-molecular processes, such as direct dissociation, pre-dissociation and ultrafast internal conversion (IC), etc., have long been studied with various methods. In polyatomic molecules and clusters, IC plays a dominant role in the deactivation process of the electronic excited states. The dynamics of ultrafast IC have received increasingly detailed experimental [1–4] and theoretical [5] attention in recent years. Multiphoton excitation methods, especially the REMPI technique, have provided rich information of structure and dy-
*
Corresponding author. Fax: +86-411-4675584. E-mail address: [email protected] (L. Wang).
namics of a wide range of electronically excited molecules [6]. Before the advent of ultrashort laser pulses, natural line width measurement has been usually used to estimate the lifetime of excited states. In order to extract the natural line width from excitation spectra, several conditions must be satisfied to avoid spectral broadening caused by sources other than natural lifetime. First and most important, the spectral width of the excitation light must be smaller than the natural line width. Secondly, mechanisms such as pressure, Doppler, and laser power broadening must be preferably excluded or corrected for. Thirdly, to determine the exact line width, a specific line shape has to be assumed for the spectral line, which introduces uncertainties in the lifetime estimation. Furthermore, it is not always possible to observe an
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 2 5 5 - 5
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S.-H. Yin et al. / Chemical Physics Letters 356 (2002) 227–232
individual line as the result of blending of several rotational lines. It is often extremely difficult to measure the exact lifetimes reliably via the natural line widths. A lower limit for the lifetime, on the other hand, in most cases, can be obtained without difficulty. The rapid development of femtosecond technology has made it possible to measure the lifetime of excited states directly in real-time [1]. The REMPI technique, in combination with the femtosecond pump-probe method, becomes an excellent tool to investigate dynamics in excited states. One of the most interesting molecular systems is ammonia. Ground state ammonia is pyramidal, possessing C3v geometry with a ð1a1 Þ2 ð2a1 Þ2 ð1eÞ4 ð3a1 Þ2 orbital configuration. Inversion doubling gives rise to ground state levels which are either symmetric or antisymmertric with respect to inversion: these are best considered in terms of the D3h point group as being of A01 or A002 vibronic symmetry. The inversion splitting in the zero-point vibrational levels is 0:793 cm1 [7,8], which is far too small to be resolved in femtosecond experimental study. Thus the molecule behaves as if it has D3h ground state geometry. The electronic excited states of ammonia are formed by promoting an electron from the doubly occupied 3a1 in C3v or 1a002 in D3h nitrogen lone-pair orbital to Rydberg orbital with principal quantum number n P 3, resulting in planar (D3h ) equilibrium geometries. Each electronic transition appears with a long vibrational progression associated with excitation of the out-of-plane mode m02 [8,9]. All of the known excited Rydberg states of ammonia are, to some extent, pre-dissociated [10] and show dynamical decay behavior on a time scale ranging from femtoseconds to nanoseconds. The pre-dissociation lifetime depends sensitively on the initially prepared level [11–14]. ~ 1 A00 of ammonia has The first Rydberg state A 2 been extensively studied both theoretically and ~ 1 A00 ðm0 Þ pre-dissoexperimentally [8,11–19]. The A 2 2 ciated by tunneling effect through a barrier whose height depends on the out-of-plane angle and increases rapidly with increasing deviation from planarity [8,15]. ~ 0 1 A0 state of ammonia was firstly adThe E 1 dressed by Colson et al. and assigned as 4pz a002 orbital with A01 symmetry [20]. The rotational
~ 0 1 A0 state band contour and constants of the E 1 ~ 0 1 A0 ð4pa002 3a1 Þ were similar to those in the C 1 state ð3pa002 3a1 Þ [9,21]. Ashfold et al. [9] studied ~ 0 1 A0 Rydberg states of ammonia by using the E 1 two-photon REMPI at excitation wavelengths in ~ 0 1 A0 state pre-dissothe range 275–248 nm. The E 1 ~ 0 1 A0 ciated in the similar mechanism with the C 1 state [21,26], i.e., by non-adiabatic coupling with ~ 1 A00 state. high vibrational levels of the A 2 In this paper, we report our time-resolved ~ 0 1 A0 state measurement of decay dynamics of the E 1 of ammonia using femtosecond (2 þ 10 ) multiphoton ionization. This is the first femtosecond twocolor real-time measurement for this state. We attempt to understand the de-excitation dynamics ~ 0 1 A0 state and interpret the results with of the E 1 pre-dissociation and IC mechanism.
2. Experimental The experimental setup is illustrated in Fig. 1. Our homemade solid-state femtosecond laser system consists of an oscillator, an amplifier and non-
Fig. 1. Schematic diagram of the optical setup (DC: dichroic mirror, BBO I: type I BBO crystal, BBO II: type II BBO crystal; SHG/THG: second/third harmonic generation; L1, L2: fused silicon lens).
S.-H. Yin et al. / Chemical Physics Letters 356 (2002) 227–232
linear optics. The oscillator is a 5 W Arþ laser (Innova 300, Coherent, full emission wavelengths) pumped self-mode-lock Ti:sapphire laser ð20–30 fs, 86 MHz, centered at 800 nm). Output of this oscillator is directed into a regenerative Ti:sapphire amplifier pumped by 3 kHz Nd:YAG laser (5 mJ per pulse at 532 nm). The final output from the amplifier is a 3 kHz train of fundamental pulses (80 fs, 800 nm, 200 lJ per pulse, 400–600 cm1 bandwidth). The fundamental light is frequency doubled by a b-BaB2 O4 crystal (BBO, type I) to produce the second harmonic generation (SHG) pulse, centered at about 400 nm. The output of BBO I is separated into SHG and fundamental lines. These two beams are then overlapped spatially and temporally into another BBO crystal (type II) to generate the third harmonic generation (THG). The residual SHG in the BBO II output is then directed into a delay-stage (SIGMA KOKI SP26150), while the THG light is introduced into timeof-flight mass spectrometer (TOF MS) via 29 cm focus lens and fused silicon window. The timedelayed SHG is focused by 26 cm focus lens and collinearly aligned with THG into TOF MS. Typical power values of the SHG and THG beams in front of the window are about 20 and 2–3 lJ, respectively. The TOF MS is a homemade Wiley-McLaren type machine. It consists of beam source and main chamber, both of which are pumped by turbo pumps. Pure ammonia gas is introduced into the beam chamber via a vitreous capillary array (Hamamatsu J5022-01) in the form of continuous leak. A small amount of benzene was added for calibration. The distance from the nozzle to the interaction region is about 2 cm. The field-free flight length is about 39 cm. Ions are detected by a Z-stack micro-channel plate (MCP) detector at the end of TOF MS. Ion signals are recorded and averaged by a computer controlled transient digitizer (STR81G, Sonix) with 1 GHz sampling rate.
ion signal, is used to determine the cross-correlation function between pump and probe light. This method has been developed by Radloff et al. [22,23]. Two SHG photons just excite the benzene molecules to its S2 electronic state [3,24,25]. The short lifetime of S2 [25] (about 50 fs) makes it possible to extract the exact cross-correlation from the decay curve. C6 Hþ 6 decay trace in our pumpprobe experiment is shown in Fig. 2. The decay curve is the convolution result of lifetime and cross-correlation function of pump light (SHG) and probe light (THG). Providing Gaussian functions for both laser pulses, the cross-correlation is then determined as 160 2 fs in our experiment. The lifetime 53 3 fs is identical with the literature report [25]. A typical time-of-flight mass spectrum of ammonia obtained in our experiment is illustrated in Fig. 3. No dimer ions could be found in the spectrum. The capillary array injection is also helpful to avoid the dimer generation. The energy scheme of ammonia is shown in Fig. ~ 1 A00 state and 4. Two SHG photons could reach A 2 ~ 0 1 A0 two THG photons would excite ammonia to E 1 state. Absorption of another THG or SHG would ionize ammonia molecule. The ammonia parent ion yield as a function of the delay time is shown in Fig. 5. The solid lines in Fig. 5a, b are convolution results of exponential decay and cross-correlation. In Fig. 5a, we merely suppose Gaussian shape function for cross-correlation and two exponential decay components in
3. Results and discussion The resonant ionization of the benzene, which leads to a narrow pump-probe decay curve for the
229
Fig. 2. Decay curve of benzene.
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(a)
Fig. 3. TOF mass spectrum of ammonia.
(b) Fig. 5. Decay curve of ammonia. Positive delay direction indicated THG as pump and SHG as probe. Solid lines represented best fitting results: (a) simulated without decay component in the negative delay direction; (b) simulated with decay component in the negative delay direction.
Fig. 4. Energy scheme of ammonia.
the positive delay direction, which means THG as pump and SHG as probe. In Fig. 5b, we add another exponential decay component in the negative delay direction, which means SHG as pump and THG as probe. Checking these two figures, one
could find out that fitting in Fig. 5b was better than in Fig. 5a. What happened in the negative and positive delay direction? In the negative delay direction, SHG light was used as pump source and THG as probe. Two ~ 1 A00 SHG photons would excite ammonia to A 2 state as illustrated in Fig. 4. The energy gap between vibrational energy levels in this state is about 900 cm1 [14,20]. The band positions for ~ 1 A00 state were assigned at m02 ¼ 3; 4; 5 levels of A 2 48 869, 49 783, 50 730 cm1 , respectively [14]. Therefore, for two SHG photons excitation (centered at 5000 cm1 with 400–600 cm1 bandwidth), only m02 ¼ 4 could be dominantly populated. Pre-dissociation of this state could be simulated by an exponential decay component with the time constant of 51 4 fs. This result agrees quite well with previous measurement [11].
S.-H. Yin et al. / Chemical Physics Letters 356 (2002) 227–232
For the positive delay time direction, the decay curve could be well fit by double exponential decay components, characterized by the lifetime constants as 60 4 and 936 92 fs, respectively. According to the reported spectroscopy of ammonia in this excitation region [9,20], the vibrational en~ 0 1 A0 state of ammonia is more than ergy gap for E 1 900 cm1 . The band positions for m02 ¼ 1; 2 and 3 ~ 0 1 A0 state are at 73 876, 74 798 and levels of the E 1 75 747 cm1 , respectively [9]. Hence, only m02 ¼ 2 level could be addressed in the excitation of two THG photons (centered at 74 900 cm1 with 400–600 cm1 bandwidth). ~ 0 1 A0 state was suggested to pre-dissociate The E 1 by non-adiabatic coupling with high vibrational ~ 1 A00 state [9,21]. The slow component, levels of A 2 with lifetime 936 92 fs, in Fig. 5 (positive delay direction), might be due to radiationless transition ~ 0 1 A0 ðm0 ¼ 2Þ state to high vibrational process of E 2 1 1 ~ A00 state [8,15–19]. The fast component level of A 2 (60 4 fs in Fig. 5) monitored the fast pre-disso~ 1 A00 level. The detected ciation process in this A 2 ammonia ion signal primarily corresponds to the ~ 0 1 A01 ðm0 ¼ 2Þ state, because of the fast decay rate E 2 ~ 1 A00 state. Pre-dissociation product, NH2 , of A 2 could not be detected in our experiment.
4. Conclusions By two-color femtosecond pump-probe experiments, we measure the decay dynamics of ~ 0 1 A0 ðm0 ¼ 2Þ state of ammonia. E ~ 0 1 A0 ðm0 ¼ 2Þ E 1 1 2 2 state is prepared by absorption of two femtosecond THG photons, pre-dissociated in about 1 ps ~ 1 A002 state foland non-adiabatic coupled with A ~ 1 A00 state lowed by rapid pre-dissociation of A 2 (characterized by lifetime 60 4 fs). Within ex~ 0 1 A0 ðm0 ¼ 2Þ perimental error, this lifetime of E 2 1 state agrees well with the previous report, derived by Ashfold et al. [9]. This is the first real-time ~ 0 1 A0 ðm0 ¼ 2Þ state of ammeasurement for the E 1 2 monia. Meanwhile, we obtained the lifetime of the ~ 1 A00 state of 51 4 fs, which agreed quite well A 2 with the previous measurements. Femtosecond photoelectron spectrum investigation is in progress ~ 0 1 A0 to get direct insight into the dynamics of the E 1 state of ammonia.
231
Acknowledgements This work was supported by NKBRSF (No. G 1999075300) and NSFC foundation (No. 29833080, 20003012, and 29973044).
References [1] A.H. Zewail, Femtochemistry: Ultrafast Dynamics of the Chemical Bond, World Scientific, Singapore, 1994. [2] W. Radloff, Th. Freudenberg, H.H. Ritze, V. Stert, K. Weyers, F. Noack, Chem. Phys. Lett. 245 (1995) 400. [3] W. Radloff, Th. Freudenberg, H.-H. Ritze, V. Stert, F. Noack, I.V. Hertel, Chem. Phys. Lett. 261 (1996) 301. [4] D.R. Cyr, C.C. Hayden, J. Chem. Phys. 104 (1996) 771. [5] M. Seel, W. Domcke, J. Chem. Phys. 95 (1991) 7806. [6] J.W. Hudgens, in: S.H. Lin (Ed.), Advance in Multiphoton Processes and Spectroscopy, vol. 4, World Scientific, Singapore, 1988, p. 171. [7] G. Herzberg, Electronic Spectra and Electronic Structure of Polyatomic Molecules, Van Nostrand, Princeton, 1966. [8] M.N.R. Ashfold, C.L. Bennett, R.N. Dixon, Chem. Phys. 93 (1985) 293. [9] M.N.R. Ashfold, C.M. Western, J.W. Hudgens, R.D. Johnson III, Chem. Phys. Lett. 260 (1996) 27. [10] M.N.R. Ashfold, D.H. Mordaunt, S.H.S. Wilson, in: D.C. Nechers, D.H. Volman, V. Biinavl (Eds.), Advances in Photochemistry, vol. 21, W. Ley, New York, 1996, p. 217, and references therein. [11] L.D. Ziegier, J. Chem. Phys. 82 (1985) 664. [12] J. Xie, G. Sha, X. Zhang, C. Zhang, Chem. Phys. Lett. 124 (1986) 99. [13] L.D. Ziegier, J. Chem. Phys. 86 (1987) 1703. [14] V. Vaida, M.I. McCarthy, P.C. Engelking, P. Rosmus, H.J. Werner, P. Botschwina, J. Chem. Phys. 86 (1987) 6669. [15] M.I. McCarthy, P. Rosmus, H.-J. Werner, P. Botschwina, V. Vaida, J. Chem. Phys. 86 (1987) 6693. [16] J. BieSner, L. Schnieder, J. Schmeer, G. Ahlers, X. Xie, K.H. Welge, M.N.R. Ashfold, R.N. Dixon, J. Chem. Phys. 88 (1988) 3607. [17] Y. Endo, M. Iida, Y. Ohshima, Chem. Phys. Lett. 174 (1990) 401. [18] D.H. Mordaunt, N.R. Ashfold, M.R.N. Dixon, J. Chem. Phys. 104 (1996) 6460. [19] D.H. Mordaunt, R.N. Dixon, M.N.R. Ashfold, J. Chem. Phys. 104 (1996) 6472. [20] J.H. Glownia, S.J. Riley, S.D. Colson, G.C. Nieman, J. Chem. Phys. 73 (1980) 4296. [21] M.N.R. Ashfold, R.N. Dixon, R.J. Stickland, Chem. Phys. 88 (1984) 463.
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[22] Th. Freudenberg, V. Stert, W. Radloff, J. Ringling, J. Gudde, G. Korn, I.V. Hertel, Chem. Phys. Lett. 269 (1997) 523. [23] P. Farmanara, V. Stert, H.-H. Ritze, W. Radloff, J. Chem. Phys. 113 (2000) 1705. [24] A. Hiraya, K. Shobatake, J. Chem. Phys. 94 (1991) 7700.
[25] W. Radloff, V. Stert, Th. Freudenberg, I.V. Hertel, C. Jouvet, C. Dedonder-Lardenx, D. Solgadi, Chem. Phys. Lett. 281 (1997) 20. [26] A.M. Quinton, J.P. Simons, J. Chem. Faraday Trans. II 78 (1982) 1261.
Chemical Physics Letters 356 (2002) 227–232 www.elsevier.com/locate/cplett
Femtosecond (2 þ 10) pump-probe mass spectroscopy on the Rydberg states of ammonia Shu-Hui Yin, Hong-Ping Liu, Jian-Yang Zhang, Bo Jiang, Da-Li Xu, Li Wang *, Guo-He Sha, Nan-Quan Lou State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China Received 28 September 2001; in final form 13 February 2002
Abstract The Rydberg state decay dynamics of ammonia is investigated in real-time by (2 þ 10 ) femtosecond pump-probe ~ 0 1 A0 state is determined as 936 92 fs for the first time. The ionization detection. The lifetime of ammonia in the E 1 ~ 0 1 A0 state has been discussed in this report. Concurrently, internal conversion (IC) pre-dissociation mechanism of the E 1 1 00 0 ~ the A A2 state ðm2 ¼ 4Þ lifetime is estimated as 51 4 fs, which is identical with the previous reports. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Fast intra-molecular processes, such as direct dissociation, pre-dissociation and ultrafast internal conversion (IC), etc., have long been studied with various methods. In polyatomic molecules and clusters, IC plays a dominant role in the deactivation process of the electronic excited states. The dynamics of ultrafast IC have received increasingly detailed experimental [1–4] and theoretical [5] attention in recent years. Multiphoton excitation methods, especially the REMPI technique, have provided rich information of structure and dy-
*
Corresponding author. Fax: +86-411-4675584. E-mail address: [email protected] (L. Wang).
namics of a wide range of electronically excited molecules [6]. Before the advent of ultrashort laser pulses, natural line width measurement has been usually used to estimate the lifetime of excited states. In order to extract the natural line width from excitation spectra, several conditions must be satisfied to avoid spectral broadening caused by sources other than natural lifetime. First and most important, the spectral width of the excitation light must be smaller than the natural line width. Secondly, mechanisms such as pressure, Doppler, and laser power broadening must be preferably excluded or corrected for. Thirdly, to determine the exact line width, a specific line shape has to be assumed for the spectral line, which introduces uncertainties in the lifetime estimation. Furthermore, it is not always possible to observe an
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 2 5 5 - 5
228
S.-H. Yin et al. / Chemical Physics Letters 356 (2002) 227–232
individual line as the result of blending of several rotational lines. It is often extremely difficult to measure the exact lifetimes reliably via the natural line widths. A lower limit for the lifetime, on the other hand, in most cases, can be obtained without difficulty. The rapid development of femtosecond technology has made it possible to measure the lifetime of excited states directly in real-time [1]. The REMPI technique, in combination with the femtosecond pump-probe method, becomes an excellent tool to investigate dynamics in excited states. One of the most interesting molecular systems is ammonia. Ground state ammonia is pyramidal, possessing C3v geometry with a ð1a1 Þ2 ð2a1 Þ2 ð1eÞ4 ð3a1 Þ2 orbital configuration. Inversion doubling gives rise to ground state levels which are either symmetric or antisymmertric with respect to inversion: these are best considered in terms of the D3h point group as being of A01 or A002 vibronic symmetry. The inversion splitting in the zero-point vibrational levels is 0:793 cm1 [7,8], which is far too small to be resolved in femtosecond experimental study. Thus the molecule behaves as if it has D3h ground state geometry. The electronic excited states of ammonia are formed by promoting an electron from the doubly occupied 3a1 in C3v or 1a002 in D3h nitrogen lone-pair orbital to Rydberg orbital with principal quantum number n P 3, resulting in planar (D3h ) equilibrium geometries. Each electronic transition appears with a long vibrational progression associated with excitation of the out-of-plane mode m02 [8,9]. All of the known excited Rydberg states of ammonia are, to some extent, pre-dissociated [10] and show dynamical decay behavior on a time scale ranging from femtoseconds to nanoseconds. The pre-dissociation lifetime depends sensitively on the initially prepared level [11–14]. ~ 1 A00 of ammonia has The first Rydberg state A 2 been extensively studied both theoretically and ~ 1 A00 ðm0 Þ pre-dissoexperimentally [8,11–19]. The A 2 2 ciated by tunneling effect through a barrier whose height depends on the out-of-plane angle and increases rapidly with increasing deviation from planarity [8,15]. ~ 0 1 A0 state of ammonia was firstly adThe E 1 dressed by Colson et al. and assigned as 4pz a002 orbital with A01 symmetry [20]. The rotational
~ 0 1 A0 state band contour and constants of the E 1 ~ 0 1 A0 ð4pa002 3a1 Þ were similar to those in the C 1 state ð3pa002 3a1 Þ [9,21]. Ashfold et al. [9] studied ~ 0 1 A0 Rydberg states of ammonia by using the E 1 two-photon REMPI at excitation wavelengths in ~ 0 1 A0 state pre-dissothe range 275–248 nm. The E 1 ~ 0 1 A0 ciated in the similar mechanism with the C 1 state [21,26], i.e., by non-adiabatic coupling with ~ 1 A00 state. high vibrational levels of the A 2 In this paper, we report our time-resolved ~ 0 1 A0 state measurement of decay dynamics of the E 1 of ammonia using femtosecond (2 þ 10 ) multiphoton ionization. This is the first femtosecond twocolor real-time measurement for this state. We attempt to understand the de-excitation dynamics ~ 0 1 A0 state and interpret the results with of the E 1 pre-dissociation and IC mechanism.
2. Experimental The experimental setup is illustrated in Fig. 1. Our homemade solid-state femtosecond laser system consists of an oscillator, an amplifier and non-
Fig. 1. Schematic diagram of the optical setup (DC: dichroic mirror, BBO I: type I BBO crystal, BBO II: type II BBO crystal; SHG/THG: second/third harmonic generation; L1, L2: fused silicon lens).
S.-H. Yin et al. / Chemical Physics Letters 356 (2002) 227–232
linear optics. The oscillator is a 5 W Arþ laser (Innova 300, Coherent, full emission wavelengths) pumped self-mode-lock Ti:sapphire laser ð20–30 fs, 86 MHz, centered at 800 nm). Output of this oscillator is directed into a regenerative Ti:sapphire amplifier pumped by 3 kHz Nd:YAG laser (5 mJ per pulse at 532 nm). The final output from the amplifier is a 3 kHz train of fundamental pulses (80 fs, 800 nm, 200 lJ per pulse, 400–600 cm1 bandwidth). The fundamental light is frequency doubled by a b-BaB2 O4 crystal (BBO, type I) to produce the second harmonic generation (SHG) pulse, centered at about 400 nm. The output of BBO I is separated into SHG and fundamental lines. These two beams are then overlapped spatially and temporally into another BBO crystal (type II) to generate the third harmonic generation (THG). The residual SHG in the BBO II output is then directed into a delay-stage (SIGMA KOKI SP26150), while the THG light is introduced into timeof-flight mass spectrometer (TOF MS) via 29 cm focus lens and fused silicon window. The timedelayed SHG is focused by 26 cm focus lens and collinearly aligned with THG into TOF MS. Typical power values of the SHG and THG beams in front of the window are about 20 and 2–3 lJ, respectively. The TOF MS is a homemade Wiley-McLaren type machine. It consists of beam source and main chamber, both of which are pumped by turbo pumps. Pure ammonia gas is introduced into the beam chamber via a vitreous capillary array (Hamamatsu J5022-01) in the form of continuous leak. A small amount of benzene was added for calibration. The distance from the nozzle to the interaction region is about 2 cm. The field-free flight length is about 39 cm. Ions are detected by a Z-stack micro-channel plate (MCP) detector at the end of TOF MS. Ion signals are recorded and averaged by a computer controlled transient digitizer (STR81G, Sonix) with 1 GHz sampling rate.
ion signal, is used to determine the cross-correlation function between pump and probe light. This method has been developed by Radloff et al. [22,23]. Two SHG photons just excite the benzene molecules to its S2 electronic state [3,24,25]. The short lifetime of S2 [25] (about 50 fs) makes it possible to extract the exact cross-correlation from the decay curve. C6 Hþ 6 decay trace in our pumpprobe experiment is shown in Fig. 2. The decay curve is the convolution result of lifetime and cross-correlation function of pump light (SHG) and probe light (THG). Providing Gaussian functions for both laser pulses, the cross-correlation is then determined as 160 2 fs in our experiment. The lifetime 53 3 fs is identical with the literature report [25]. A typical time-of-flight mass spectrum of ammonia obtained in our experiment is illustrated in Fig. 3. No dimer ions could be found in the spectrum. The capillary array injection is also helpful to avoid the dimer generation. The energy scheme of ammonia is shown in Fig. ~ 1 A00 state and 4. Two SHG photons could reach A 2 ~ 0 1 A0 two THG photons would excite ammonia to E 1 state. Absorption of another THG or SHG would ionize ammonia molecule. The ammonia parent ion yield as a function of the delay time is shown in Fig. 5. The solid lines in Fig. 5a, b are convolution results of exponential decay and cross-correlation. In Fig. 5a, we merely suppose Gaussian shape function for cross-correlation and two exponential decay components in
3. Results and discussion The resonant ionization of the benzene, which leads to a narrow pump-probe decay curve for the
229
Fig. 2. Decay curve of benzene.
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(a)
Fig. 3. TOF mass spectrum of ammonia.
(b) Fig. 5. Decay curve of ammonia. Positive delay direction indicated THG as pump and SHG as probe. Solid lines represented best fitting results: (a) simulated without decay component in the negative delay direction; (b) simulated with decay component in the negative delay direction.
Fig. 4. Energy scheme of ammonia.
the positive delay direction, which means THG as pump and SHG as probe. In Fig. 5b, we add another exponential decay component in the negative delay direction, which means SHG as pump and THG as probe. Checking these two figures, one
could find out that fitting in Fig. 5b was better than in Fig. 5a. What happened in the negative and positive delay direction? In the negative delay direction, SHG light was used as pump source and THG as probe. Two ~ 1 A00 SHG photons would excite ammonia to A 2 state as illustrated in Fig. 4. The energy gap between vibrational energy levels in this state is about 900 cm1 [14,20]. The band positions for ~ 1 A00 state were assigned at m02 ¼ 3; 4; 5 levels of A 2 48 869, 49 783, 50 730 cm1 , respectively [14]. Therefore, for two SHG photons excitation (centered at 5000 cm1 with 400–600 cm1 bandwidth), only m02 ¼ 4 could be dominantly populated. Pre-dissociation of this state could be simulated by an exponential decay component with the time constant of 51 4 fs. This result agrees quite well with previous measurement [11].
S.-H. Yin et al. / Chemical Physics Letters 356 (2002) 227–232
For the positive delay time direction, the decay curve could be well fit by double exponential decay components, characterized by the lifetime constants as 60 4 and 936 92 fs, respectively. According to the reported spectroscopy of ammonia in this excitation region [9,20], the vibrational en~ 0 1 A0 state of ammonia is more than ergy gap for E 1 900 cm1 . The band positions for m02 ¼ 1; 2 and 3 ~ 0 1 A0 state are at 73 876, 74 798 and levels of the E 1 75 747 cm1 , respectively [9]. Hence, only m02 ¼ 2 level could be addressed in the excitation of two THG photons (centered at 74 900 cm1 with 400–600 cm1 bandwidth). ~ 0 1 A0 state was suggested to pre-dissociate The E 1 by non-adiabatic coupling with high vibrational ~ 1 A00 state [9,21]. The slow component, levels of A 2 with lifetime 936 92 fs, in Fig. 5 (positive delay direction), might be due to radiationless transition ~ 0 1 A0 ðm0 ¼ 2Þ state to high vibrational process of E 2 1 1 ~ A00 state [8,15–19]. The fast component level of A 2 (60 4 fs in Fig. 5) monitored the fast pre-disso~ 1 A00 level. The detected ciation process in this A 2 ammonia ion signal primarily corresponds to the ~ 0 1 A01 ðm0 ¼ 2Þ state, because of the fast decay rate E 2 ~ 1 A00 state. Pre-dissociation product, NH2 , of A 2 could not be detected in our experiment.
4. Conclusions By two-color femtosecond pump-probe experiments, we measure the decay dynamics of ~ 0 1 A0 ðm0 ¼ 2Þ state of ammonia. E ~ 0 1 A0 ðm0 ¼ 2Þ E 1 1 2 2 state is prepared by absorption of two femtosecond THG photons, pre-dissociated in about 1 ps ~ 1 A002 state foland non-adiabatic coupled with A ~ 1 A00 state lowed by rapid pre-dissociation of A 2 (characterized by lifetime 60 4 fs). Within ex~ 0 1 A0 ðm0 ¼ 2Þ perimental error, this lifetime of E 2 1 state agrees well with the previous report, derived by Ashfold et al. [9]. This is the first real-time ~ 0 1 A0 ðm0 ¼ 2Þ state of ammeasurement for the E 1 2 monia. Meanwhile, we obtained the lifetime of the ~ 1 A00 state of 51 4 fs, which agreed quite well A 2 with the previous measurements. Femtosecond photoelectron spectrum investigation is in progress ~ 0 1 A0 to get direct insight into the dynamics of the E 1 state of ammonia.
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Acknowledgements This work was supported by NKBRSF (No. G 1999075300) and NSFC foundation (No. 29833080, 20003012, and 29973044).
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