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Energy transfer of ethyl iodine studied by time-resolved photoelectron imaging
Chemical Physics Letters 554 (2012) 53–56
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Energy transfer of ethyl iodine studied by time-resolved photoelectron imaging Yanqi Xu, Xuejun Qiu, Bumaliya Abulimiti, Yanmei Wang, Ying Tang, Bing Zhang ⇑ State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
a r t i c l e
i n f o
Article history: Received 8 June 2012 In final form 12 October 2012 Available online 23 October 2012
a b s t r a c t Ultrafast dynamics of electronically excited states in ethyl iodine is studied using femtosecond timeresolved photoelectron imaging coupled with mass spectroscopy. The dissociation constant of the A band was measured to be about 57 fs. Upon two 400 nm photon excitation to the B band, the time evolution of the parent ion with consists of two components. The fast component with a time constant of 50 fs revealed the energy transfer from the higher Rydberg states to the B band. The slow one was determined to be 1.42 ps, which was due to predissociation relaxation from the B band to the repulsive A band. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction During the past decades, one of the primary aims in the field of reaction dynamics was detecting and observing ultrafast processes which take place on the femtosecond time scale, such as making or breaking molecular bonds, geometrical rearrangements after energy deposition in real time [1]. The advent of ultrafast laser pulses makes it possible. In its relatively short history, femtosecond timeresolved spectroscopy, which involves a pump–probe framework, has primarily changed our viewpoint on many elementary reactions in chemistry, physics, and biology [2–4]. In femtosecond pump–probe process, a reaction, a nonstationary state or wave packet is initiated by the ultrafast pump pulse. And the evolution is monitored as a function of time by means of a suitable probe pulse. Taking account of this viewpoint, femtosecond time-resolved photoelectron spectroscopy is a useful technique to investigate excited state dynamic in isolated polyatomic molecules because of the sensitivity to both electronic configurations and vibrational dynamics. As a variant of femtosecond time-resolved photoelectron spectroscopy, femtosecond timeresolved photoelectron imaging (TRPEI) has the advantage of providing both the energy and the angular distribution of the photoelectron as well as their correlation as a function of time. It allows for the study of electronic relaxation dynamics in isolated molecules, as well as study of sequential ultrafast electronic processes in both optically bright and dark states. Alkyl halides have been concerned to the atmospheric chemistry due to their role in ozone depletion. The photodissociation of ethyl iodide (C2H5I) has been widely studied [5–15]. The first spectroscopic studies of ethyl iodide were performed in the 1930s. Since then, the UV photo absorption spectra and molar extinction coefficients have been reported between 3.7 and 14.2 eV. The
⇑ Corresponding author. Fax: +86 (0)27 87198491. E-mail address: [email protected] (B. Zhang). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.10.041
spectra have been interpreted mainly in terms of electronic band origins and vibrational levels for the lowest energy Rydberg states. The UV absorption spectra of ethyl iodine are similar to other alkyl iodides, which show a relative weak broadband at low excitation energies centered at 260 nm, traditionally labeled as the A band. The A band is assigned to the electron promotion from n into the antibonding orbital r⁄ (C–I). The electronic states formed in this r⁄ n transition are dissociative along the C–I bond. Another strong structured band, labeled as the B band, is centered at 200 nm in the UV absorption spectra of ethyl iodine. It is a Rydberg state arising from electronic transitions from a 5p iodine orbit to a higher atomic Rydberg orbital 6s of the iodine atom. Most of these gas phase studies, however, have concentrated on the direct dissociation of the A-band. The B state and higher Rydberg states, are seldom discussed. In the present Letter, we explore the ultrafast dynamics of ethyl iodine by femtosecond TRPEI. We focused primarily on timeresolved photoelectron kinetic energy (PKE) distributions to investigate dynamics from the B state and the higher Rydberg states as well as the dynamics processes of energy transfer from the higher Rydberg states to the B band and predissociation from the B band to the repulsive A band.
2. Experimental details The experimental setup has been described elsewhere [16–20] and will only be mentioned briefly here. It consists of an amplified Ti:sapphire femtosecond laser system, a molecular beam apparatus, a time-of-flight mass spectrometer (TOF-MS), and a twodimensional (2D) position sensitive detector. A differentially pumped vacuum chamber, which is divided into two chambers by a 1 mm thick skimmer, is kept under a background pressure of 4 106 Pa. The liquid sample (Ethyl iodide, 99% purity), 5% seeded in helium carrier gas at a background pressure of 2 atm, was expanded through a pulsed valve to generate a pulsed
Y. Xu et al. / Chemical Physics Letters 554 (2012) 53–56
molecular beam. The beam was skimmed and introduced into the ionization chamber. The 360 mm field-free region of the TOF spectrometer is shielded by a l-metal tube to avoid external magnetic fields that might otherwise deflect the electronic trajectories. The generated photoelectrons were extracted and accelerated by the electrostatic immersion lens and were projected onto a 2D detector comprising a microchannel plate/phosphor screen and a chargecoupled device camera. The observed images of the electrons were inverted to generate the speed and angular distributions by the basis-set expansion method (BASEX). The details of our femtosecond laser system have been described elsewhere [21]. It consists of an oscillator and a chirped regenerative amplifier with a stretcher and compressor. It generates a 1 kHz pulse train centered at 800 nm of 45 fs pulse width with maximum energy of 1 mJ/pulse. The 800 nm pulse is used as probe pulse. The second harmonic pulse (400 nm) is generated in a 0.5 mm thick BBO crystal. The third harmonic pulse (267 nm) is generated in a 0.2 mm thick BBO crystal by sum frequency mixing of the second harmonic pulse and the fundamental pulse. And, the fourth harmonic pulse (200 nm) is generated in another 0.2 nm thick BBO crystal by sum frequency mixing of the third harmonic pulse and the fundamental pulse. The pump and probe beams with vertical polarization were merged by a dichroic mirror and directed perpendicularly into the molecular beam chamber. The time delay between the pump and probe pulses were accurately monitored by a computer-controlled linear translation stage (PI, M-126.CG1).
3. Results and discussion In ethyl iodide, the maximum of the total A state absorption corresponding to the n ? r⁄ transition is peaked around 38 800 cm1. The relative contributions of the individual components to the overall absorption cross section are 3Q0 (81%, maximum at 38 820 cm1, full width at half maximum 5060 cm1), 3 Q1 (9%, 36 500 cm1, fwhm 4060 cm1) and 1Q1 (10%, 40 275 cm1, fwhm 3530 cm1) [22]. Herein, the molecules are pumped to their A band by a single photon of 267 nm. The corresponding delay-time constant is yielded to be s = 57 fs from the time-resolved total ion signal of the parent ion emerged after one photon absorption of the 267 nm pulse as a function of the pump–probe delay, which reflects the excitation and decay of the initially excited molecules in A band. In order to excite the molecules to the B band, the laser pulse of 400 nm was chosen. Two photons are needed since the B band start near 200 nm. Figure 1 shows the total ion signals, integrated over all recoil speeds, measured for C2H5I+ and C2H5+, respectively, as a function of the pump–probe delays. The decay profiles are fitted with the sum of decaying exponential convolved with Gaussian describing the instrument response function. The decay profiles of both C2H5I+ and C2H5+ are fitted by two components. And both of them have a fast decay as well as a relative slow decay. All fragments are from the dissociation of C2H5I+ parent ions and have no contribution to the photoelectron signal. This will be discussed later. Figure 2 shows typical photoelectron images measured at a series of delay times between the pump and probe pulses while using 400 nm as pump pulse. These images are the inverse Abel transforms of the observed data, representing a section of the 3D photoelectron scattering distribution. Five well-resolved concentric rings were observed in the images, which are obvious especially in the image acquired at 110 fs delay time. The rings with different radii stand for different photoelectron kinetic energy components. The corresponding photoelectron energy distributions are shown in Figure 3. These five peaks are observed centered at 0.65, 0.94, 1.26, 2.23 and 2.79 eV, and they are assigned as the first, second, third, fourth and fifth peak labeled from the inside to outside,
(a) C2H5I
+
τ1 = 50 fs τ2 = 1.42 ps
Ion Intensity(a.u.)
54
(b) C2H5
+
τ3 = 50 fs τ4 = 1.03 ps
0
1
2 3 Delay time(ps)
4
Figure 1. The C2H5I+ and C2H5+ ion signals recorded as a function of delay time between the pump pulse at 400 nm and the probe pulse at 800 nm. Circles indicate the experimental data. The best fits (solid lines) yield the corresponding time constants.
respectively. It can be seen that the decay times of the ion counts for peaks 1 and 3 are obviously longer than the other three ones. Different decay times of these peaks suggested that they may have different production mechanisms. Peaks 1 and 3 can be explained by a (2 + 3) (two pump and three probe photons) ionization scheme via the B state. The molecules are excited to the B state after two-photon absorption of the pump pulse and then ionized by three probe photons. These two peaks correspond to C2H5I+ in two spin–orbit states, (2E3/2) and (2E1/2) whose ionization potentials are 9.35 and 9.93 eV, respectively [8]. The calculated available energies (maximum of the released kinetic energy) of this ionization scheme agree well with our experimental data of 0.65 and 1.26 eV. Similarly, Peaks 4 and 5 can be explained by a (3 + 2) (three pump and two probe photons) scheme via higher Rydberg states or (2 + 4) (two pump and four photons) scheme. While, if peaks 4 and 5 are from (2 + 4) scheme, the decay time of these two peaks should be as that of peaks 1 and 3 since they undergo the same electronic state. Since their evolution is different from that of peaks 1 and 3, peaks 4 and 5 probably correspond to the evolution of higher Rydberg states via a 3 + 2 excitation scheme. This is likely due to the high power of the 400 nm pump as observed by the 2 photon excitation. The lifetime fitted for the time-dependent ion signals of C2H5I+ are s1 = 50 fs and s2 = 1.42 ps, respectively. The slow one (s2 = 1.42 ps) reflects the excitation and decay of the initially excited B band. Considering the 6 nm bandwidth of our pump laser, the band origin 000, the C–I stretching mode m10 are conceivably involved [22]. The 1.42 ps maybe is a colligation lifetime of these two possible intermediate states. The possible decay pathway is attributed to predissociating to the band A. The time constant is similar with CH3I [18], and the small peak 3 may come from predissociation. The faster lifetime s1 may reflect the excitation and decay of the higher Rydberg states such as 7de, etc. [15] which
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Y. Xu et al. / Chemical Physics Letters 554 (2012) 53–56
1.0
0.5
0.0 lay
14 0 16 0 40 0 64 94 0 0
De
11
0 0
Relative intensity (a.u.)
Figure 2. A series of time-resolved photoelectron images (inverse-Abel transform) of C2H5I observed with a pump laser wavelength of 400 nm and a probe wavelength of 800 nm at different delay time. The linear polarizations of the pump and probe lasers are aligned vertical in the plane of the figure.
tim
e(
fs)
1 2 3 Photoelectron energy (eV)
Figure 3. The photoelectron energy distributions at different pump–probe delays with a pump laser wavelength of 400 nm. Five peaks centered at 0.65, 0.94, 1.26, 2.23 and 2.79 eV are observed.
Relative intensity(a.u.)
1.0
110fs 140fs 160fs
0.8 0.6 0.4 0.2
can be pumped by three photons of 400 nm. In the traditional concept, Rydberg states have long lifetime. Nevertheless, such short lifetimes have also been observed in some special Rydberg states of several molecular systems. The Rydberg states of CH3I and CD3I were studied [23,24] using two-photon femtosecond excitation, the dynamics of the 310 transitions of the 6p state, and the 000 transition of the 7s state were determined, the decay time were s (6p310) = 141 ± 3 fs, and s (7s000) = 115 ± 11 fs. In order to confirm that the faster component is not from the multiphoton absorption, we use 200 nm to pump and 400 nm to probe. The corresponding delay-time constant is yielded to be 1.05 ps. The absence of the ultrafast component helps to confirm that the short lived states must be populated by three 400 nm photons, which cannot be reached by a 200 nm photon. The corresponding two delay-time constants of C2H5+ are yielded to be s3 = 50 fs and s4 = 1.03 ps, respectively. We believe that C2H5+ ions come from the dissociation of parent C2H5I+ ions, because the time-dependent ion signals of C2H5I+ and C2H5+ have the same trend. The total energy in the previously mentioned (2 + 3) and (3 + 2) scheme all exceed the appearance potential of 10.44 eV [24] of C2H5+. If they are from the neutral dissociation of excited ethyl iodine to ionize the neutral C2H5 radical whose IP is 8.12 eV [25], six probe photons are needed, respectively. This scheme is unfavorable in the energy view. Also, the neutral dissociation channel required to produce the C2H5 radicals is competitive with the ionization process to produce C2H5I+. In this case, the time-dependent ion signals of C2H5+ should have opposite trends with C2H5I+. Thus, it is reasonable that C2H5+ comes from the dissociation of C2H5I+. The parent ion absorbed more photons of probe pulse and was initially pumped into a dissociative or predissociative state or formed in a bound state, but later suffers a further promotion into a dissociative state. It is just like CH3I studied by Banares et al. [26]. The process can be represented as below scheme.
C2 H5 I ! C2 H5 Iþ þ e ;
0.0 0
1 2 3 Photoelectron energy(eV)
4
Figure 4. The photoelectron energy distributions at some special pump–probe delays with a pump laser wavelength of 400 nm extracted from the Figure 3.
C2 H5 Iþ ! C2 Hþ5 þ I
The lifetime difference between them may come from different involved intermediate vibration modes. The dissociation of parent ions into C2H5+ may only have correspondence with some special mode, while the appearance of C2H5I+ is a colligation effect of several intermediate vibration modes.
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To gain further insight into the changes of peaks with the pump–probe delay time, we choose the PKE at some special delays which have obvious changes. The fourth and fifth peaks decay rapidly with increasing time, revealing that the populated Rydberg states undergo a very rapid decay pathway. While the first and third peaks increase with delay time within the time window shown in Figure 4. It indicates that there exists population transfer between peaks 1, 3 and peaks 4, 5. These phenomena reveal that upon excitation into the higher Rydberg states, the molecules undergo a rapid energy transfer to the B band. The time constant of the energy transfer is about 50 fs (from 110 to 160 fs). It accords with the short lifetime component of C2H5I+ and C2H5+. We suggest that the decay way of the higher Rydberg states is a rapid transfer to the B band. 4. Conclusion In summary, we have observed the ultrafast processes arising from optically excited states of ethyl iodine. Using 267 nm pulse to pump, the dissociated band A state is excited and the time constant is determined to be 57 fs according to the decay of the ion signals. By two-photon absorption at 400 nm, the B band is excited and the lifetime of predissociation is determined to be 1.42 ps. For higher Rydberg states excited by pump pulse at 400 nm with three photons, a rapid shift of electrons from the higher Rydberg states to the B band is observed by time-resolved PKE distributions, which shows energy transfer processes on real time. And its time constant is approximately 50 fs. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 20903116, 91121006 and 21173256)
and Scientific Research Foundation for the Chinese Academy of Sciences Dean Award Winner. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
I.V. Hertel, W. Radloff, Rep. Prog. Phys. 69 (2006) 1897. A.H. Zewail, J. Phys. Chem. A 104 (2000) 5660. M. Dantus, A.H. Zewail, Chem. Rev. 104 (2004) 1717. A.W. Castleman, M.L. Kimble, Femtochemistry VII: Fundamental Ultrafast Processes in Chemistry, Physics, and Biology, Elsevier, New York, 2006. J.R. Cao, Y.H. Huang, D.L. Yang, Chin. J. Chem. Phys. 3 (1990) 235. I. Kovacs, F. Solymosi, J. Chem. Phys. 97 (1993) 11056. W.K. Kang, K.W. Jung, D.C. Kim, Chem. Phys. 196 (1995) 363. N. Knoblauch, A. Strobel, J. Chem. Phys. 103 (1995) 5417. O.V. Rattigan, D.E. Shallcross, R.A. Cox, J. Chem. Soc. Faraday Trans. 93 (1997) 2839. D.X. Dai, C. Tao, J.H. Zhang, Abstr. Pap. Am. Chem. Soc. 218 (1999) 386. P. Brewer, P. Das, G.S. Ondrey, J. Chem. Phys. 79 (1983) 720. F.G. Godwin, P.A. Gorry, P.M. Hughes, Chem. Phys. Lett. 135 (1987) 163. W.B. Bi, X.L. Xu, J.G. Huang, D.Q. Xiao, Q.H. Zhu, Sci. China Ser. B: Chem. 50 (2007) 476. Y. Tang, W.B. Lee, Z.F. Hu, B. Zhang, K.C. Lin, J. Chem. Phys. 126 (2007) 064302. A. Giulian, J. Chem. Phys. 110 (1999) 10307. Y.Z. Liu, B.F. Tang, H. Shen, S. Zhang, B. Zhang, Opt. Express 18 (2010) 5791. Q.S. Zheng, C.C. Qin, J.Y. Long, B.F. Tang, S. Zhang, B. Zhang, Sci. China 53 (2010) 1040. Z.Z. Cao, Z.R. Wei, L.Q. Hua, C.J. Hu, S. Zhang, B. Zhang, J. Chem. Phys. 130 (2009) 144309. Y.M. Wang, H. Shen, L.Q. Hua, C.J. Hu, B. Zhang, Opt. Express 17 (2009) 10506. Z.Z. Cao, Z.R. Wei, L.Q. Hua, C.J. Hu, S. Zhang, B. Zhang, Chem. Phys. Chem. 10 (2009) 1299. Z.R. Wei, F. Zhang, Y.M. Wang, B. Zhang, Chin. J. Chem. Phys. 20 (2007) 419. A. Gedanken, Chem. Phys. Lett. 137 (1987) 462. M. Dantus, M.H.M. Janssen, A.H. Zewail, Chem. Phys. Lett. 181 (1991) 281. M.H.M. Janssen, M. Dantus, H. Guo, A.H. Zewail, Chem. Phys. Lett. 214 (1993) 281. J.C. Traeger, R.G. McLoughlin, J. Am. Chem. Soc. 103 (1981) 3647. J. Durá, R. de Nalda, G.A. Amaral, L. Bañares, J. Chem. Phys. 131 (2009) 134311.
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Energy transfer of ethyl iodine studied by time-resolved photoelectron imaging Yanqi Xu, Xuejun Qiu, Bumaliya Abulimiti, Yanmei Wang, Ying Tang, Bing Zhang ⇑ State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
a r t i c l e
i n f o
Article history: Received 8 June 2012 In final form 12 October 2012 Available online 23 October 2012
a b s t r a c t Ultrafast dynamics of electronically excited states in ethyl iodine is studied using femtosecond timeresolved photoelectron imaging coupled with mass spectroscopy. The dissociation constant of the A band was measured to be about 57 fs. Upon two 400 nm photon excitation to the B band, the time evolution of the parent ion with consists of two components. The fast component with a time constant of 50 fs revealed the energy transfer from the higher Rydberg states to the B band. The slow one was determined to be 1.42 ps, which was due to predissociation relaxation from the B band to the repulsive A band. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction During the past decades, one of the primary aims in the field of reaction dynamics was detecting and observing ultrafast processes which take place on the femtosecond time scale, such as making or breaking molecular bonds, geometrical rearrangements after energy deposition in real time [1]. The advent of ultrafast laser pulses makes it possible. In its relatively short history, femtosecond timeresolved spectroscopy, which involves a pump–probe framework, has primarily changed our viewpoint on many elementary reactions in chemistry, physics, and biology [2–4]. In femtosecond pump–probe process, a reaction, a nonstationary state or wave packet is initiated by the ultrafast pump pulse. And the evolution is monitored as a function of time by means of a suitable probe pulse. Taking account of this viewpoint, femtosecond time-resolved photoelectron spectroscopy is a useful technique to investigate excited state dynamic in isolated polyatomic molecules because of the sensitivity to both electronic configurations and vibrational dynamics. As a variant of femtosecond time-resolved photoelectron spectroscopy, femtosecond timeresolved photoelectron imaging (TRPEI) has the advantage of providing both the energy and the angular distribution of the photoelectron as well as their correlation as a function of time. It allows for the study of electronic relaxation dynamics in isolated molecules, as well as study of sequential ultrafast electronic processes in both optically bright and dark states. Alkyl halides have been concerned to the atmospheric chemistry due to their role in ozone depletion. The photodissociation of ethyl iodide (C2H5I) has been widely studied [5–15]. The first spectroscopic studies of ethyl iodide were performed in the 1930s. Since then, the UV photo absorption spectra and molar extinction coefficients have been reported between 3.7 and 14.2 eV. The
⇑ Corresponding author. Fax: +86 (0)27 87198491. E-mail address: [email protected] (B. Zhang). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.10.041
spectra have been interpreted mainly in terms of electronic band origins and vibrational levels for the lowest energy Rydberg states. The UV absorption spectra of ethyl iodine are similar to other alkyl iodides, which show a relative weak broadband at low excitation energies centered at 260 nm, traditionally labeled as the A band. The A band is assigned to the electron promotion from n into the antibonding orbital r⁄ (C–I). The electronic states formed in this r⁄ n transition are dissociative along the C–I bond. Another strong structured band, labeled as the B band, is centered at 200 nm in the UV absorption spectra of ethyl iodine. It is a Rydberg state arising from electronic transitions from a 5p iodine orbit to a higher atomic Rydberg orbital 6s of the iodine atom. Most of these gas phase studies, however, have concentrated on the direct dissociation of the A-band. The B state and higher Rydberg states, are seldom discussed. In the present Letter, we explore the ultrafast dynamics of ethyl iodine by femtosecond TRPEI. We focused primarily on timeresolved photoelectron kinetic energy (PKE) distributions to investigate dynamics from the B state and the higher Rydberg states as well as the dynamics processes of energy transfer from the higher Rydberg states to the B band and predissociation from the B band to the repulsive A band.
2. Experimental details The experimental setup has been described elsewhere [16–20] and will only be mentioned briefly here. It consists of an amplified Ti:sapphire femtosecond laser system, a molecular beam apparatus, a time-of-flight mass spectrometer (TOF-MS), and a twodimensional (2D) position sensitive detector. A differentially pumped vacuum chamber, which is divided into two chambers by a 1 mm thick skimmer, is kept under a background pressure of 4 106 Pa. The liquid sample (Ethyl iodide, 99% purity), 5% seeded in helium carrier gas at a background pressure of 2 atm, was expanded through a pulsed valve to generate a pulsed
Y. Xu et al. / Chemical Physics Letters 554 (2012) 53–56
molecular beam. The beam was skimmed and introduced into the ionization chamber. The 360 mm field-free region of the TOF spectrometer is shielded by a l-metal tube to avoid external magnetic fields that might otherwise deflect the electronic trajectories. The generated photoelectrons were extracted and accelerated by the electrostatic immersion lens and were projected onto a 2D detector comprising a microchannel plate/phosphor screen and a chargecoupled device camera. The observed images of the electrons were inverted to generate the speed and angular distributions by the basis-set expansion method (BASEX). The details of our femtosecond laser system have been described elsewhere [21]. It consists of an oscillator and a chirped regenerative amplifier with a stretcher and compressor. It generates a 1 kHz pulse train centered at 800 nm of 45 fs pulse width with maximum energy of 1 mJ/pulse. The 800 nm pulse is used as probe pulse. The second harmonic pulse (400 nm) is generated in a 0.5 mm thick BBO crystal. The third harmonic pulse (267 nm) is generated in a 0.2 mm thick BBO crystal by sum frequency mixing of the second harmonic pulse and the fundamental pulse. And, the fourth harmonic pulse (200 nm) is generated in another 0.2 nm thick BBO crystal by sum frequency mixing of the third harmonic pulse and the fundamental pulse. The pump and probe beams with vertical polarization were merged by a dichroic mirror and directed perpendicularly into the molecular beam chamber. The time delay between the pump and probe pulses were accurately monitored by a computer-controlled linear translation stage (PI, M-126.CG1).
3. Results and discussion In ethyl iodide, the maximum of the total A state absorption corresponding to the n ? r⁄ transition is peaked around 38 800 cm1. The relative contributions of the individual components to the overall absorption cross section are 3Q0 (81%, maximum at 38 820 cm1, full width at half maximum 5060 cm1), 3 Q1 (9%, 36 500 cm1, fwhm 4060 cm1) and 1Q1 (10%, 40 275 cm1, fwhm 3530 cm1) [22]. Herein, the molecules are pumped to their A band by a single photon of 267 nm. The corresponding delay-time constant is yielded to be s = 57 fs from the time-resolved total ion signal of the parent ion emerged after one photon absorption of the 267 nm pulse as a function of the pump–probe delay, which reflects the excitation and decay of the initially excited molecules in A band. In order to excite the molecules to the B band, the laser pulse of 400 nm was chosen. Two photons are needed since the B band start near 200 nm. Figure 1 shows the total ion signals, integrated over all recoil speeds, measured for C2H5I+ and C2H5+, respectively, as a function of the pump–probe delays. The decay profiles are fitted with the sum of decaying exponential convolved with Gaussian describing the instrument response function. The decay profiles of both C2H5I+ and C2H5+ are fitted by two components. And both of them have a fast decay as well as a relative slow decay. All fragments are from the dissociation of C2H5I+ parent ions and have no contribution to the photoelectron signal. This will be discussed later. Figure 2 shows typical photoelectron images measured at a series of delay times between the pump and probe pulses while using 400 nm as pump pulse. These images are the inverse Abel transforms of the observed data, representing a section of the 3D photoelectron scattering distribution. Five well-resolved concentric rings were observed in the images, which are obvious especially in the image acquired at 110 fs delay time. The rings with different radii stand for different photoelectron kinetic energy components. The corresponding photoelectron energy distributions are shown in Figure 3. These five peaks are observed centered at 0.65, 0.94, 1.26, 2.23 and 2.79 eV, and they are assigned as the first, second, third, fourth and fifth peak labeled from the inside to outside,
(a) C2H5I
+
τ1 = 50 fs τ2 = 1.42 ps
Ion Intensity(a.u.)
54
(b) C2H5
+
τ3 = 50 fs τ4 = 1.03 ps
0
1
2 3 Delay time(ps)
4
Figure 1. The C2H5I+ and C2H5+ ion signals recorded as a function of delay time between the pump pulse at 400 nm and the probe pulse at 800 nm. Circles indicate the experimental data. The best fits (solid lines) yield the corresponding time constants.
respectively. It can be seen that the decay times of the ion counts for peaks 1 and 3 are obviously longer than the other three ones. Different decay times of these peaks suggested that they may have different production mechanisms. Peaks 1 and 3 can be explained by a (2 + 3) (two pump and three probe photons) ionization scheme via the B state. The molecules are excited to the B state after two-photon absorption of the pump pulse and then ionized by three probe photons. These two peaks correspond to C2H5I+ in two spin–orbit states, (2E3/2) and (2E1/2) whose ionization potentials are 9.35 and 9.93 eV, respectively [8]. The calculated available energies (maximum of the released kinetic energy) of this ionization scheme agree well with our experimental data of 0.65 and 1.26 eV. Similarly, Peaks 4 and 5 can be explained by a (3 + 2) (three pump and two probe photons) scheme via higher Rydberg states or (2 + 4) (two pump and four photons) scheme. While, if peaks 4 and 5 are from (2 + 4) scheme, the decay time of these two peaks should be as that of peaks 1 and 3 since they undergo the same electronic state. Since their evolution is different from that of peaks 1 and 3, peaks 4 and 5 probably correspond to the evolution of higher Rydberg states via a 3 + 2 excitation scheme. This is likely due to the high power of the 400 nm pump as observed by the 2 photon excitation. The lifetime fitted for the time-dependent ion signals of C2H5I+ are s1 = 50 fs and s2 = 1.42 ps, respectively. The slow one (s2 = 1.42 ps) reflects the excitation and decay of the initially excited B band. Considering the 6 nm bandwidth of our pump laser, the band origin 000, the C–I stretching mode m10 are conceivably involved [22]. The 1.42 ps maybe is a colligation lifetime of these two possible intermediate states. The possible decay pathway is attributed to predissociating to the band A. The time constant is similar with CH3I [18], and the small peak 3 may come from predissociation. The faster lifetime s1 may reflect the excitation and decay of the higher Rydberg states such as 7de, etc. [15] which
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Y. Xu et al. / Chemical Physics Letters 554 (2012) 53–56
1.0
0.5
0.0 lay
14 0 16 0 40 0 64 94 0 0
De
11
0 0
Relative intensity (a.u.)
Figure 2. A series of time-resolved photoelectron images (inverse-Abel transform) of C2H5I observed with a pump laser wavelength of 400 nm and a probe wavelength of 800 nm at different delay time. The linear polarizations of the pump and probe lasers are aligned vertical in the plane of the figure.
tim
e(
fs)
1 2 3 Photoelectron energy (eV)
Figure 3. The photoelectron energy distributions at different pump–probe delays with a pump laser wavelength of 400 nm. Five peaks centered at 0.65, 0.94, 1.26, 2.23 and 2.79 eV are observed.
Relative intensity(a.u.)
1.0
110fs 140fs 160fs
0.8 0.6 0.4 0.2
can be pumped by three photons of 400 nm. In the traditional concept, Rydberg states have long lifetime. Nevertheless, such short lifetimes have also been observed in some special Rydberg states of several molecular systems. The Rydberg states of CH3I and CD3I were studied [23,24] using two-photon femtosecond excitation, the dynamics of the 310 transitions of the 6p state, and the 000 transition of the 7s state were determined, the decay time were s (6p310) = 141 ± 3 fs, and s (7s000) = 115 ± 11 fs. In order to confirm that the faster component is not from the multiphoton absorption, we use 200 nm to pump and 400 nm to probe. The corresponding delay-time constant is yielded to be 1.05 ps. The absence of the ultrafast component helps to confirm that the short lived states must be populated by three 400 nm photons, which cannot be reached by a 200 nm photon. The corresponding two delay-time constants of C2H5+ are yielded to be s3 = 50 fs and s4 = 1.03 ps, respectively. We believe that C2H5+ ions come from the dissociation of parent C2H5I+ ions, because the time-dependent ion signals of C2H5I+ and C2H5+ have the same trend. The total energy in the previously mentioned (2 + 3) and (3 + 2) scheme all exceed the appearance potential of 10.44 eV [24] of C2H5+. If they are from the neutral dissociation of excited ethyl iodine to ionize the neutral C2H5 radical whose IP is 8.12 eV [25], six probe photons are needed, respectively. This scheme is unfavorable in the energy view. Also, the neutral dissociation channel required to produce the C2H5 radicals is competitive with the ionization process to produce C2H5I+. In this case, the time-dependent ion signals of C2H5+ should have opposite trends with C2H5I+. Thus, it is reasonable that C2H5+ comes from the dissociation of C2H5I+. The parent ion absorbed more photons of probe pulse and was initially pumped into a dissociative or predissociative state or formed in a bound state, but later suffers a further promotion into a dissociative state. It is just like CH3I studied by Banares et al. [26]. The process can be represented as below scheme.
C2 H5 I ! C2 H5 Iþ þ e ;
0.0 0
1 2 3 Photoelectron energy(eV)
4
Figure 4. The photoelectron energy distributions at some special pump–probe delays with a pump laser wavelength of 400 nm extracted from the Figure 3.
C2 H5 Iþ ! C2 Hþ5 þ I
The lifetime difference between them may come from different involved intermediate vibration modes. The dissociation of parent ions into C2H5+ may only have correspondence with some special mode, while the appearance of C2H5I+ is a colligation effect of several intermediate vibration modes.
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To gain further insight into the changes of peaks with the pump–probe delay time, we choose the PKE at some special delays which have obvious changes. The fourth and fifth peaks decay rapidly with increasing time, revealing that the populated Rydberg states undergo a very rapid decay pathway. While the first and third peaks increase with delay time within the time window shown in Figure 4. It indicates that there exists population transfer between peaks 1, 3 and peaks 4, 5. These phenomena reveal that upon excitation into the higher Rydberg states, the molecules undergo a rapid energy transfer to the B band. The time constant of the energy transfer is about 50 fs (from 110 to 160 fs). It accords with the short lifetime component of C2H5I+ and C2H5+. We suggest that the decay way of the higher Rydberg states is a rapid transfer to the B band. 4. Conclusion In summary, we have observed the ultrafast processes arising from optically excited states of ethyl iodine. Using 267 nm pulse to pump, the dissociated band A state is excited and the time constant is determined to be 57 fs according to the decay of the ion signals. By two-photon absorption at 400 nm, the B band is excited and the lifetime of predissociation is determined to be 1.42 ps. For higher Rydberg states excited by pump pulse at 400 nm with three photons, a rapid shift of electrons from the higher Rydberg states to the B band is observed by time-resolved PKE distributions, which shows energy transfer processes on real time. And its time constant is approximately 50 fs. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 20903116, 91121006 and 21173256)
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