Chapter 4. He(I) photoelectron imaging of benzene and pyridine
4.2 He(I) ultraviolet photoelectron imaging of pyridine
4.2.1 Introduction
Pyridine (C5H5N) is an important model system in relation to biologically active nicotinic acid and the nucleotides of cytosine, uracil, and thymine. It is also an example of heterocyclic aromatic molecules. The replacement of a carbon atom with a nitrogen atom in a benzene ring creates a lone-pair orbital. The lowest photoelectron band of pyridine has
These early photoelectron experiments suggested all three possible assignments of
--n,1, 35 -n-,36 and n--37 - 40. The resolution of this confusion required additional information. Utsunomiya et al.41 performed the first angle-resolved PES of pyridine in 1978 and found that near the ionization threshold increases from 0.2 to ~ 0.6 in the middle of the first band. They referred that ionization to the n-1 state of 2,6-lutidine (dimethyl-pyridine)
90 in agreement with experimental studies. MRDCI predict that the first two ionic states have an energy difference of 0.7 eV, whereas the other methods predict differences less than 0.2 eV.
So far, angle-resolved PES of pyridine has been limited to the first three ionic states. In this study, the energy-dependent anisotropy parameter is determined for the entire energy region accessible with He(I) radiation.
4.2.2 Results
Figure 4.3 (a) shows a photoelectron image measured with a pulsed supersonic beam of 10 % pyridine seeded in He. The background image has been subtracted. The left half is the raw image and the right half is the slice image obtained by inverse Abel transform. Figure 4.3 (b) shows the photoelectron energy spectrum and extracted from the slice image. The numerical values of are also compared with the literature41,42 in Table 4.2. A microdischarge of MCP occurred when a high density molecular beam impinged on the detector, we needed to restrict the stagnation pressure for supersonic expansion to be 0.2 atm. (The observed microdischarge was a specific problem of this detector.) Because of the low stagnation pressure of 0.2 atm, a long (3 hours) integration time was needed to measure the image. In the future, microdischarge should be avoided by redesigning the PEI spectrometer so that a molecular beam travells parallel to the face of the imaging detector.
As seen in Fig. 4.3 (b), of the first band between 9.2 and 10.2 eV increases with the binding energy in agreement with Utsunomiya et al.41 and Piancastelli et al.,42. This band has
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contribution of D0 and D1 associated with electron removal from 11a1(n) and 1a2() orbitals, respectively. In He(I) UPS of pyridine, the first band corresponds to PKE between 12.02 and 11.02 eV. In this PKE region, increases only gradually with PKE if it is due to the energy-dependent Coulomb phases.31 The observed increases rather rapidly with the electron binding energy (i.e. decreasing PKE), which indicates that the variation of in the first band is not due to Coulomb phases and rather due to D1 overlap with D0. The origin of D1 has not been determined yet.
The fourth and fifth bands between 12.5 and 13.5 eV have been assigned to ionization from b2 and b1, respectively.41 As far as we know, has not been determined at the electron binding energies over 14 eV. The variation of in this region is similar to that of benzene; has a deep minimum at about 15.8 eV for pyridine and 14.9 eV for benzene. All these cation states are due to the removal of an electron from the orbitals (see Fig. 4.3). beam by introduction of a pulsed beam. The major source of the background photoelectrons are from residual water vapor in the photoioniztion chamber. The photoelectron anisotropy parameters determined for benzene and pyridine were in good agreement with the literatures.
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The anisotropy parameters were determined for the first time for pyridine at the electron binding energies over 14 eV. Higher energy resolution is obtainable in principle by centroiding calculations of the light spots on the phosphor screen of the detector. However, our super-resolution imaging system is currently able to handle up to 256 light spots in a single frame (30 frames/s), which is too low for He(I) PEI. This problem will be solved in the future when a higher frame rate of a camera and a faster digital image processing circuit become available.
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Figure 4.1 (a) Photoelectron image of a continuous supersonic beam of 8% benzene seeded in He (the background image has been subtracted). (b) Ion image with the supersonic molecular beam. (c) Ion image without the molecular beam. The arrow indicates the propagation direction of the He(I) radiation.
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Figure 4.2 (a) Symmetrized photoelectron image of a pulsed supersonic beam of benzene. (b) the slice image obtained from (a). (c) The energy-dependent anisotropy parameters (upper panel, solid circle) and photoelectron spectra (lower panel) obtained from (b). Data from [ref.
12], [ref. 19] and [ref. 11] are given as open squares (□), open triangles () and open inverted triangles (), respectively. The assignments of the ionized orbitals are indicated at the vertical ionization point [ref. 19]. The error bars are the fitting errors.
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Figure 4.3 (a) Symmetrized photoelectron image of a pulsed supersonic beam of 10%
pyridine seeded in He (left half) and the slice image taking the inverse Abel transform (right half). (b) The energy-dependent anisotropy parameter (upper panel, solid circles) and photoelectron spectra (lower panel) obtained from (a). Data from [ref. 42] and [ref.41] are given as open squares (□) and open triangles (), respectively. The assignments of the ionized orbitals are indicated at the vertical ionization point [ref. 44]. The error bars are the fitting errors.
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Table 4.1 Electron binding energy (in eV) and anisotropy parameters () for benzene
This study Sell et al.c Carlson et al.d Mattsson et al.f
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98
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Table 4.1 (Continued)
This study Sell et al. Carlson et al. Mattsson et al.
Orbital eBE eBE eBE eBE
19.13 0.09(5)
19.19 0.16(5) 19.20 0.06(12)e
19.25 0.15(5) 19.31 0.09(6) 19.37 0.03(6)
aThe orbital assignments for benzene are those in ref. [19]. bErrors (, in last digit unless indicated otherwise) given in parentheses. cThe energy-dependent anisotropy parameters are reproduced from ref. [25]. dThe energy-dependent anisotropy parameters are reproduced from ref. [19]. e was determined by integrating over the area of the 2e2g() band. fReference [24].
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Table 4.2 Electron binding energy (in eV) and anisotropy parameters () for pyridine.
this study Utsunomiya et al.c Piancastelli et al.d
orbitala eBE eBE eBE
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Table 4.2 (Continued)
this study Utsunomiya et al. Piancastelli et al.
orbital eBE eBE eBE
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Table 4.2 (Continued)
this study Utsunomiya et al. Piancastelli et al.
orbital eBE eBE eBE
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aThe orbital assignments for benzene are those made in ref. [44]. bErrors (, in last digit unless indicated otherwise) given in parentheses. cReference [41]. dThe energy-dependent anisotropy parameters are reproduced from ref. [42].
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4.4 References
1 Turner, D. W. et al. Molecular Photoelectron Spectroscopy, Wiley-Interscience, London, 1970.
2 Asbrink, L.; Edqvist, E.; Lindholm, E.; Selin, L. E. Chem. Phys. Lett. 1970, 5, 192.
3 Asbrink, L.; Lindholm, E.; Edqvist, O. Chem. Phys. Lett. 1970, 5, 609.
4 Carlson T. A.; Anderson, C. P. Chem. Phys. Lett. 1971, 10, 561.
5 Potts, A. W. et al. Faraday Discuss. Chem. Soc. 1972, 54, 168.
6 Lindholm, E. Faraday Discuss. Chem. Soc. 1972, 54, 200.
7 Clark, P. A.; Brogli, F.; Heilbronner, E. Helv. Chim. Acta 1972, 55, 1415.
8 Streets, d. G.; Potts, A. W. J. Chem. Soc. Faraday Trans. II 1974, 70 ,1505.
9 Kobayashi, T.; Nagakura, S. J. Electron Spectrosc. Relat. Phenom. 1975, 16, 221.
10 Karsson, L. et al. Physica scripta 1976, 14, 230.
11 Mattsson, l. et al. Physica scripta 1977, 16, 149.
12 Sell, J. A.; Kupperman, A. Chem. Phys. 1978, 33, 367.
13 Bieri, G.; Asbrink, L. J. Electron Spectrosc. Relat. Phenom. 1980, 20, 149.
14 Kimura, K. et al. Handbook of HeI Photoelectron Spectra of Fundamental Organic Molecules, Japan Scientific Societies press, Tokyo, 1981.
15 Gelius, U. et al. J. Electron Spectrosc. Relat. Phenom. 1974, 2, 405.
16 Gelius, U. et al. J. Electron Spectrosc. Relat. Phenom 1974, 5, 985.
17 Kinsinger, J. A.; Taylor, J. W. Int. J. Mass Spectrom. Ion Phys. 1972, 10, 445.
18 Mehaffy, D. et al. J. Electron Spectrosc. Relat. Phenom. 1983, 28, 239.
19 Carlson, T. A. et al. J. Chem. Phys. 1987, 86, 6918.
20 Baltzer, P. et al. Chem. Phys. 1997, 224, 95.
21 Gelius, U. et al. Phys. Scr. 1971, 3, 237.
22 Brundle, C. R.; Robin, M. B.; Kuebler, N. A. J. Am. Chem. Soc. 1972, 94, 1466.
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29 Krause, H.; Ernstberger, B.; Neusser, H. J. Chem. Phys. Lett. 1991, 184, 411.
30 Shiromaru, H.; Katsumata, S. Bull. Chem. Soc. Jpn. 1984, 57, 3543.
31 Suzuki, Y.; Suzuki, T. J. Phys. Cham. A 2008, 112, 402.
32 EL Sayed, M. F. A.; Kasha, M.; Tanaka, Y. J. Chem. Phys. 1961, 34, 334.
33 Jonsson, B. O.; Lindholm, E.; Skerbele, A. Int. J. Mass Spectrom. Ion Phys. 1969 3, 385.
34 Jacqueline, O. Berg; Parker, D. H.; El-Sayed, M. A. Chem. Phys. Lett. 1978, 56, 411.
35 Baker, A. D.; Turner, D. W. Phil. Trans. Roy. Soc. London. Ser. A 1970, 268, 131.
36 Dewar, M. J. S.; Worley, S. D. J. Chem Phys. 1969, 51, 263.
37 Gleiter, R.; Heilbronner, E.; Hornung, V. Angew. Chem. 1970, 82, 878.
38 Asbrink, L.; Fridh, C.; Lindholm, E. Chem. Phys. Lett. 1977, 52, 69.
39 Karlsson, L. et al. Chem. Scr 1974 6, 214.
40 Gleiter, R.; Heilbronner, E. Hornug, V. Helv. Chim. Acta 1972, 55, 255.
41 Utsumoniya, C.; Kobayashi, T. Nagakura, S. Bull. Chem. Soc. Jpn. 1978, 51, 3482.
42 Piancastelli, M. N. et al. J. Am. Chem. Soc. 1983, 105, 4235.
43 Moghaddam, M. S. et al. Chem. Phys. 1996, 207, 19.
44 Wan, J.; Hada, M.; Ehara, M.; Nakatsuji, H. J. Chem. Phys. 2001, 114, 5117.
45 Plashkevych, O. et al. J. Electron Spectrosc. Relat. Phenom. 2000, 106, 51.
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46 Lorentzon, J.; Fülscher, M. P.; Roos, B. O. Theor. Chim. Acta 1995, 92, 67.
47 Walker, I. C.; Palmer, M. H.; Hopkirk, A. Chem. Phys. 1990, 141, 365.
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Chapter 5
Time-resolved PEI of Pyrazine using a fs-UV Laser and a VUV FEL
The first time-resolved photoelectron imaging using a vacuum ultraviolet free-electron laser and a femtosecond ultraviolet laser is presented. The key instrument for this achievement is the photoelectron imaging spectrometer developed using He(I) light source.
Ultrafast internal conversion (IC) and intersystem crossing (ISC) in pyrazine in a supersonic molecular beam are discussed in terms of the observed time profiles of photoelectron intensity and kinetic energy distribution.
5.1 Introduction
The ultrafast pump-probe spectroscopy (Fig. 5.1) enables us to study real-time dynamics of chemical reactions. In this method, a pump laser pulseprepares molecules in an excited electronic state and a probe pulse interrogates their time evolution with various spectroscopic methods.1 Transient absorption and fluorescence up-conversion techniques are often used as the probing method in the condensed-phase, while gas-phase studies of molecules and clusters usually use laser-induced fluorescence or photoionization (including single photon and resonant multiphoton ionization) because of their high sensitivity. However, photoionization has the following advantages over laser induced fluorescence. Firstly, ionization allows observation of dark states with extremely low fluorescence quantum yields; therefore, it enables observation of intersystem crossing (ISC) to triplet states and internal conversion (IC) to the ground electronic state. Secondly, ionization detection is extremely sensitive, since
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photoionization is an induced process and the photoelectrons can be collected efficiently by an electromagnetic field. Thus, I use pump-probe time-resolved photoelectron spectroscopy (TRPES) to study real-time dynamics of molecules.
As I have already described in earlier chapters, photoelectron imaging (PEI) enables efficient and accurate measurements of the photoelectron kinetic energy (PKE) and photoelectron angular distribution (PAD). The latter is important for elucidation of the characters of the electronic states involved in the dynamics. The time-resolved photoelectron imaging (TRPEI) has already been used extensively in studying ultrafast electronic dynamics in photoexcited molecules.2,3 However, what is novel in this work is the use of a femtosecond vacuum ultraviolet laser free electron laser (VUV-FEL) for photoionization.
It is noted that TRPES using UV photons sometimes had difficulty in observation of the excited-state molecules when they undergo molecular deformation of a large scale. For example, recent TRPES of isolated DNA bases have observed femtosecond (fs) or picosecond (ps) decays of photoionization signals, which were interpreted as ultrafast electronic deactivations; 4,5,6 however, theoretical calculations have suggested that the decays were owing to rapid increase of the effective ionization energy caused by molecular deformation that stabilize and destabilize the neutral and ionic state, respectively. TRPES with a VUV or extreme UV (EUV) probe lasers would avoid such confusion and allow clear observation of ultrafast electronic dynamics and photochemical reactions.7
The VUV light source I use in this study is VUV-FEL named SCSS (SPring-8 compact self-amplification of spontaneous emission source) at RIKEN Harima instate. SCSS is usually operated at 51–61 nm, and its maximum pulse energy and pulse duration are 30 J/pulse and sub-picosecond, respectively. On the other hand, the drawback of the VUV-FEL is its low repetition rate, 20 – 30 Hz. The advantage of PEI is that its ultimate detection solid angle of photoelectrons compensates the low repetition rate and makes possible to perform pump-probe experiments in acceptable data acquisition time (eg. one photoelectron image /
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hour).
Although pump-probe experiments using a femtosecond laser and an EUV FEL (Hamburg; FLASH) have been reported,8-12 these previous studies were limited to nonlinear optical processes of atoms induced by a Ti:sapphire laser (800 nm) and an FEL. As far as I know, the study presented in this chapter is the first UV pump – VUV-FEL probe experiment on chemical dynamics of polyatomic molecules.
5.2 One-photon ionization by SASE-FEL
5.2.1 SPring-8 Compact SASE Source
SCSS13 (Spring-8 compact self-amplification of spontaneous emission (SASE) source) was constructed as a prototype of X-ray FEL (XFEL). SCSS emits radiation in the VUV to EUV regions.14 Figure 5.2 shows a schematic configuration of SCSS that comprises of an ultralow-emittance electron gun, C-band accelerator and two-stage in-vacuum undulator. The electron beam (500 keV) from the electron gun is accelerated to 250 MeV in tandem C-band accelerators, and a beam halo is removed by a chicane. In the first section of the undulator, spontaneous emission of radiation occurs from the electrons and it serves as seed radiation for induced emission in the main part of the undulator. Thus, spontaneous emission of radiation is amplified. Since the spontaneous emission of radiation is stochastic, Self-Amplification of Spontaneous Emission of radiation (SASE) inherently has fluctuation in wavelength and energy. SASE is used because there is no good optical mirror to construct a laser cavity in the EUV to X-ray regions. The characteristics of SCSS14 are summarized as follows:
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5.2.2 Suppression of background photoemission using newly designed electrode system Since the coherent VUV-FEL beam can be focused tightly, background photoemission induced by scattered light are much less than in the case of He(I) radiation. However, we found that the conventional three-plate electrostatic lens reported by Eppink and Parker15 generates considerable background electrons even with FEL, as shown in Fig. 5.3 (a) and (b).
On the other hand, the background signal was almost completely eliminated by our new electrostatic lens, as shown in Fig. 5.3 (c): notice that it is a raw image without background subtraction. The background signal that still remains in Fig. 5.3 (c) is due to photoelectron from residual water vapor in the vacuum chamber and not background photoemission from the apparatus. Water vapor pressure can be reduced considerably by baking the entire vacuum chamber for a week; therefore, it is possible to eliminate photoelectron signals from water vapor. However, in practice, a severely limited beam time of SCSS did not allow us to bake the chamber prior to the experiment unfortunately.
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5.2.3 Comparison of PEI with He(I) and SASE-FEL
As discussed in detail in Chapter 3, the resolution of He(I) PEI is limited by a large ionization volume. On the other hand, the VUV-FEL beam can be focused to a diameter of ~ 0.1 mm. Our trajectory calculations indicate that the electron lens system is capable of providing the energy resolution E/E of 0.04 % at 5.461 eV, if an ionization volume is 0.1 mm 3.4 mm. The resolution of 0.04 % reaches the ultimate resolution, 0.06 %, obtainable with the best commercial MCP (70 mm in diameter and 10 m in pore size). The proof of the ultimately high resolution of our PEI apparatus awaits an experiment with nanosecond UV lasers.
However, as shown in Figure 5.4, photoelectron kinetic energy distributions of Kr measured with FEL radiation (wavelength 58.4 nm) is broader than that with measured He(I) radiation under the same experimental condition; the FWHM of the peak at 7.22 eV (indicated by * in Fig. 5.4) is 246 meV with He(I) and 298 meV with FEL. In both cases, a high-resolution CCD camera (2048 2048) was used without centroiding calculation. The inferior resolution with FEL, in spite of its smaller ionization volume, is ascribed to a large effective bandwidth (0.1 eV) of SASE.14
Figure 5.5 shows a photoelectron image of pyridine with FEL at 58.4 nm and the photoelectron spectra extracted from the image. Since FEL radiation is polarized, the photoelectron distribution is cylindrically symmetric about its polarization direction (indicated by the arrow in the Fig.5.5). Similarly with Kr, the image shown in Fig. 5.5 exhibit broader structures than Fig. 4.3 measured with He(I).
One of the advantages of VUV-FEL over He(I) is much higher photon density. However, the pulse energy of the SCSS radiation is too high for single-photon ionization experiments.
We had to reduce pulse energy with metal and gas filters to avoid multiphoton ionization.
Since SCSS is operated at a low repetition rate (20–30 Hz), the reduction of pulse energy diminishes the advantage of VUV-FEL over He(I) for conventional UPS. In the next section,
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we utilize another important advantage of VUV-FEL, namely the ultrashort pulse duration, for pump-probe experiments.
5.3 (1 + 1’) pump-probe experiment of pyrazine
5.3.1 Introduction
Pyrazine (C4H4N2, 1,4-diazabenzene) is a heterocyclic aromatic molecule that belongs to the D2h point group. The S2-S0 absorption spectrum of pyrazine in the deep UV region (230–
270 nm) exhibits a broad feature,16 which implies ultrafast decay of the S2 state. Low-lying conical intersection (CI) between the S2 and S1 potential energy surfaces was identified by semiempirical calculations.17 Thereafter, extensive ab initio calculations have been performed to characterize S2/S1 CI more precisely,18,19 and it has been established that the S2/S1 CI is located at the bottom of the S2 diabatic PES. Thus, the ultrafast decay from S2 has been established as the internal conversion through CI, as shown in fig. 5.6 (a). For observation of this ultrafast dynamics in real time, TRPES of pyrazine has been proposed by theoretical simulations.20 , 21 , 22
However, the time constant of the internal conversion is predicted to be only less than 30 fs.23,24 Such high time-resolution has been hardly realized in the pump-probe experiments in deep UV region. For instance, Wang et al.25 performed the first femtosecond [1+2’] TRPEI of pyrazine to study this internal conversion; however, their time-resolution (450 fs) did not allow observation of ultrafast decay from S2 to S1, and they observed only the decay of S1 after internal conversion from S2. The S1-S0 decay time constant was 22 ps. Stert et al.26 performed a similar experiment. Very recently, TRPEI of pyrazine was performed with sub-20 fs deep UV pulse in our laboratory, and the time constant of the S2S1 internal conversion was experimentally determined as 22 fs.27,28
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The S1 T1 (n *) ISC in pyrazine is also well known as the intermediate coupling case in molecular radiationless transition. Frad et al.29 and Lahmani et al.30 observed ISC as a biexponential fluorescence decay, in which the fast component corresponds to ISC. The biexponential fluorescence decay was extensively studied in the 1980s and a debate ensued as to whether the observed fast component was due to the predicted dephasing or Rayleigh–
Raman light scattering.16,31,32 The presence of a fast component and its finite lifetime have been unambiguously proved by picosecond laser spectroscopy. Although fluorescence studies only observed the decay of the singlet state, TR-PEI allowed detection of both S1 and T1 states involved in ISC in pyrazine by (1+2’) resonance enhanced multiphoton ionization (REMPI)25,33,34 and (1+1’) REMPI.35 In the latter, pyrazine was excited to 00 level of S1 with a 324 nm femtosecond pump pulse and subsequently ionized with a 197 nm femtosecond probe pulse. Ionization from the S1 zero vibrational level occurred to low vibrational states of cation, due to Franck–Condon principle, creating rapid electrons, whereas ionization from high vibrational levels of the triplet state created slow electrons, as shown in Fig. 5.6 (b).
TR-PEI clearly demonstrated a rapid decay of the ionization component from S1 and a growth of that from T1 with a clear isosbestic point in the time-dependent photoelectron spectra. The lifetime of S1 was estimated to be 110 ps.
We revisited internal conversion and ISC in pyrazine by TRPEI using VUV-FEL.
5.3.2 Experimental setup
Figure 5.7 shows the experiment setup for pump-probe experiments using a fs UV laser and a VUV FEL. The vacuum chamber and PEI spectrometer are the same as He(I) experiment described in chapters 2 and 3. A pulsed supersonic molecular beam of 10 % pyrazine (C4H4N2) seeded in He was introduced into a photoionization chamber and crossed with a femtosecond UV laser pulse and a VUV FEL pulse (161 nm). The UV laser at 324 or 260 nm excited pyrazine to the first or second excited singlet state (S1 of S2), respectively, and
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the VUV laser ionizes molecules from the excited states. Electrons generated by (1+1’) REMPI were accelerated along the molecular beam propagation axis and projected onto a position-sensitive detector comprising 10-m pore MCPs, a phosphor screen (P47), and a CCD camera (512 512 pixels). Light baffles were used at the entrance and exit ports of the laser beams to reduce scattered light.
The femtosecond laser system comprises an oscillator, a regenerative amplifier, and an optical parametric amplifier (OPA). The laser system was operated at 1 kHz, and a pulse picker was used to gate a pulse train from the OPA. Although the gate width of our pulse picker was long to allow two consecutive pulses separated by 1 ms to pass through each gate, background subtraction eliminated any interference from this additional pulse. The UV laser pulse was introduced into the vacuum chamber through an optical path length of about 10 m.
The laser beam was focused with an axisymmetric lens placed in the air and reflected with an
The laser beam was focused with an axisymmetric lens placed in the air and reflected with an