Time-resolved photoelectron spectroscopy of polyatomic molecules using 42-nm vacuum ultraviolet laser based on high harmonics generation

Accepted Manuscript Research paper Time-Resolved Photoelectron Spectroscopy of Polyatomic Molecules using 42nm Vacuum Ultraviolet Laser Based on High Harmonics Generation Junichi Nishitani, Christopher W. West, Chika Higashimura, Toshinori Suzuki PII: DOI: Reference:

S0009-2614(17)30696-6 http://dx.doi.org/10.1016/j.cplett.2017.07.025 CPLETT 34951

To appear in:

Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

16 May 2017 7 July 2017 10 July 2017

Please cite this article as: J. Nishitani, C.W. West, C. Higashimura, T. Suzuki, Time-Resolved Photoelectron Spectroscopy of Polyatomic Molecules using 42-nm Vacuum Ultraviolet Laser Based on High Harmonics Generation, Chemical Physics Letters (2017), doi: http://dx.doi.org/10.1016/j.cplett.2017.07.025

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Time-Resolved Photoelectron Spectroscopy of Polyatomic Molecules using 42-nm Vacuum Ultraviolet Laser Based on High Harmonics Generation

Junichi Nishitani, Christopher W. West, Chika Higashimura, and Toshinori Suzuki* Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-Ku, Kyoto 606-8502, Japan

Abstract Time-resolved photoelectron spectroscopy (TRPES) of gaseous polyatomic molecules using 266-nm (4.7 eV) pump and 42-nm (29.5 eV) probe pulses is presented. A 1-kHz Ti:sapphire laser with a 35 fs pulse duration is employed to generate high harmonics in Kr gas, and the 19th harmonic (42-nm) was selected using two SiC/Mg mirrors. Clear observation of the ultrafast electronic dephasing in pyrazine and photoisomerization of 1,3-cyclohexadiene demonstrates the feasibility of TRPES with the UV pump and VUV probe pulses under weak excitation conditions in the perturbation regime.

*Corresponding

author: [email protected]

1

1. Introduction Non-adiabatic transitions are ubiquitous among multi-dimensional potential energy surfaces of polyatomic molecules, and they create a variety of reaction pathways and cause branching into different products. 1, 2 For elucidating such complex chemical dynamics, it is crucial to observe non-adiabatic transitions entirely from the initial electronic state to the final products. Such observation is possible using time-resolved photoelectron spectroscopy (TRPES) with vacuum ultraviolet (VUV) radiation that enables ionization from all electronic states of the reactant, transient species and the products. Previously, we performed this type of experiment using a VUV free electron laser (SCSS: SPring-8 Compact SASE Source).1, 3 However, its low repetition rate (10 – 30 Hz) and limited beam time have hampered wide applications. More recently, we succeeded in generating the sixth (9.3 eV) and ninth (14 eV) harmonic of a Ti:sapphire laser in the laboratory using filamentation four-wave mixing4, 5 and tandem third harmonic generation (3 × 3) processes of a Ti:sapphire laser,6 respectively. Their pulse energies (nJ – J) are sufficient and their high repetition-rates are clearly advantageous for TRPES. With these laser-based VUV light sources, we performed real-time observation of the cascaded electronic deactivation7, 8 and photoisomerization reaction9 of polyatomic molecules. In the present paper, we employ high harmonics generation (HHG) for TRPES of organic molecules. HHG has been attracting much attention in recent years as a means to produce attosecond pulses and/or soft X-ray radiation.10-14 Although HHG sources have high potential as light sources for TRPES, there are numerous obstacles to their use. One is spectral isolation of a single harmonic from a number of harmonics, separated each other by 3.1 eV, without deterioration of the time-resolution. Excellent designs of grating-based monochromators have already been reported for this purpose; however, they are still complex in structure and large in size.15-19 The other is small pulse energy, on the order of ten pJ, of a single-order harmonic, with which TRPES is more difficult than with more powerful pulses generated in the UV region, especially when using weak pump pulses that excite only a few percent of the ground-state molecules to the excited electronic states. In the present paper, we implemented a compact single-order harmonic light source using HHG and SiC/Mg multi-layer mirrors20-24 to perform TRPES using UV (266 nm, 4.7 eV) pump and VUV (42 nm, 29.5 eV) probe pulses. The use of the VUV probe allows us to observe a broader spectral range due to the increased photon energy. A magnetic-bottle time-of-flight (TOF) photoelectron spectrometer is employed to analyze photoelectron kinetic energy (PKE). The results clearly demonstrate the utility of the light source, and the feasibility of TRPES of organic photochemical reactions in the gas phase. Because the sample densities of liquids and solids are higher than those of gases, TRPES of condensed matters is expected to provide higher

2

signals although the measurement should be performed such that unwanted broadening of the spectrum induced by a space charge effect is minimized.

Detector (MCP)

Delay stage

TOF

Quartz block ( 5 cm)

TMP

BBO

HWP

3

BBO

800 nm, 1.5 mJ 35 fs, 1 kHz

CP

Quartz window

Al filter ( 200 nm )

f = 500 mm e-

Skimmer

Gaseous sample

19

TMP

3/8 inch Teflon

HHG

Magnet

TMP

Quartz window

Quartz lens f = 500 mm

1/8 inch SUS

f = 1000 mm

Aperture (Φ2 mm)

TMP

1/16 inch SUS

Kr gas ( 100 torr )

800 nm, 1.0 mJ 35 fs, 1 kHz

Fig. 1 Schematic diagram of our experimental apparatus.

2. Experimental A schematic diagram of our experimental apparatus is shown in Fig. 1. A one-box Ti-sapphire regenerative amplifier (Coherent Astrella, 35 fs, 800 nm, 1 kHz, 6 mJ) was used to generate the third harmonic (3) pump pulses (266 nm, 4.7 eV) and the 19th harmonic (19) probe pulses (42 nm, 29.5 eV). A part (1.5 mJ) of the fundamental output () of the Ti:sapphire laser was frequency-tripled using a third harmonic generation setup, that consists of a 200-μm-thick β-BaB2O4 (BBO) crystal for second harmonic generation (SHG), a calcite plate (CP) for group velocity delay compensation, a 200-μm-thick half waveplate (HWP), and a 50-μm-thick β-BBO crystal for mixing of and2. The obtained pulse energy of 3 was 5 – 6 J and the pump pulse was focused by an Al concave mirror (f = 1000 mm). To ensure efficient one-photon excitation while avoiding multi-photon excitation of the sample, the pulse duration of 3was increased to about 1 ps using a quartz block, 5 cm in length. Another part (1.0 mJ) of  was focused into Kr gas cell using a quartz lens (f = 500 mm), passing across the Teflon tube through two pinholes drilled by the laser beam itself. The gas pressure of Kr at the focal point was estimated to be less than 100 torr. The high harmonics generated in the Kr gas and co-propagate in a vacuum tube, and a 200-nm-thick Al filter blocked The 19th order harmonic was selected using a concave (f = 500 mm) and a flat SiC/Mg mirror, which has a reflection peak at 29.6 eV. The diameter of VUV light at an ionization point was expected to be about 40 m in FWHM.24 The VUV photon flux was measured to be 2 × 109 photon/sec. at maximum using a calibrated VUV photodetector. The 3pump and 19probe pulses are introduced into a magnetic-bottle TOF photoelectron spectrometer with a small crossing angle

3

(1.5 ), and spatially overlapped in front of an electron-sampling skimmer for the TOF spectrometer. The overlap is optimized to the position showing the greatest enhanced signals in the pump-probe spectrum. The magnetic-bottle guided more than 50% of photoelectron emitted from the sample to the detector. The sample was kept in a stainless steel cell at room temperature, and its vapor was introduced into the photoionization chamber as an effusive jet from a PEEK tube with a 400-m inner diameter. Our TOF spectrometer is capable of changing its electric potential, and we adjusted the potential to reject strong photoelectron signals due to one-photon ionization from the ground-state molecules to the excited cationic states. Photoelectrons were detected using a Chevron microchannel plate (MCP) and a preamplifier, and electron counts were measured using a multi-channel scaler.

3. Results 3.1 Ultrafast electronic dephasing in pyrazine

(a)

0.8

3b2u,5ag 6ag

3b3g,1b3u

1b1g

0.6

4b2u

0.4 0.2 5b1u,1b2g

0.0 5

10

3b2u,5ag

(b)

1.0

Intensity (arb. units)

Intensity (arb. units)

1.0

0.8

5b1u,1b2g

0.6

4b2u 4ag,2b3g

6ag,1b1g

0.4 3b3g,1b3u

0.2

4b1u

15 20 Binding energy (eV)

0.0 25

5

10

15 20 Binding energy (eV)

25

Fig. 2 (a) Previously reported photoelectron spectrum of jet-cooled pyrazine measured using SCSS (58.4 nm, 21.2 eV).1 (b) Photoelectron spectrum of pyrazine vapor measured using 29.5 eV probe pulse in the present experiment. Figure 2(a) is the photoelectron spectrum of jet-cooled pyrazine measured using SCSS (58.4 nm, 21.2 eV) and a velocity-map photoelectron imaging apparatus.1 Figure 2(b) shows the one-photon photoelectron spectrum of pyrazine vapor measured using our 29.5 eV light source and a magnetic-bottle photoelectron spectrometer. Each band corresponds to different cationic state created by removal of an electron from different molecular orbitals. 25 We can find good agreement between the two spectra. On the other hand, as reported by Oku et al., high-resolution PFI-ZEKE spectroscopy of a jet-cooled pyrazine exhibits a large number of vibrational bands, especially for the first photoelectron band. 26 Such vibrational structure is unresolved in Fig. 2, due to the large bandwidth of VUV radiation employed in these measurements; the spectral bandwidth of our 29.5 eV light source is estimated to be 0.5 eV.

4

50 -10 ps 2 ps

0.8 0.6 1.2 0.4 1.0

(a)

1b1g 6ag

-10 ps 2 ps

0.2 0.8

0.0 0.6 0.02 0.4

(c)

50

1b1g 6ag

40

40

(a)

0.8

-10 Intensity 0 (arb. 10units)20 30 Delay time (ps)

1.0

23.0 - 24.0 eV 20.3 - 20.6 eV 19.4 - 19.7 eV

Intensity (arb. units) units) (arb. units) Intensity (arb.Intensity

1.2

0.6

30

0.4

0.2

0.02

(b)

20

0.0

(b)

-0.2

0.01

0.01 0.2

-0.4 -0.6

0.0010

0.0 0.00



n

-0.01

Hot S0

-0.01

-0.8

S1

-1.0

0

-0.02

-0.02

-0.03 -10

-60

24

-40

22 PKE (eV)

0

20

20

18

-20

-0.03

19 18

20 2021

22 22 23 2424 PKE (eV)

2526

Intensity (arb. units)

Fig. 3 (a) PKE distributions measured for pyrazine vapor at the pump-probe delay times of -10 and 2 ps. (b) Difference PKE spectrum between -10 and 2 ps. (c) Difference PKE spectrum as a function of the pump-probe delay time.

Figure 3 (a) shows the PKE distributions measured for pyrazine vapor at the pump-probe time delays of -10 and 2 ps. Each spectrum has been smoothed to see the fine structure in the difference spectrum. Since we applied a retardation potential of 18 eV to our magnetic-bottle spectrometer, the bands with PKE < 18 eV have not been observed. The two spectra measured at different time delays are quite similar. However, their difference, shown in Fig. 3 (b), clearly exhibits enhanced signals from the excited electronic states and the ground-state bleach. Figure 3 (c) presents the difference spectrum as a function of the pump-probe time delay; the red color corresponds to a positive signal enhanced in the presence of pump pulses, while the blue color shows a negative signal that diminished with the pump pulses. The pump pulse at 266 nm optically prepares S2(*), which undergoes internal conversion to S1(n,*) within 22 fs through the conical intersection of the potential energy surfaces.27 This ultrafast internal conversion process is unseen in Fig. 3, because of insufficient time-resolution in the present experiment. The S1(n,*) decays in ca. 20 ps due to internal conversion to S0 and intersystem crossing to T1(n,*), as discussed by Horio et al..7 The signal observed in the region of 23.0 – 24.5 eV in Fig. 3 (c) is the photoionization signal from S1(n,*), while the signal in the region 20.0 – 21.0 eV is of photoionization from both S 1(n,*) and S0 created by S1→S0 internal conversion. The latter appears almost constant signal intensity over time, because S1(n,*) predominantly decays to hot S0 state which has a long lifetime. Although vibrationally excited pyrazine molecules undergo unimolecular dissociation, it takes more than microseconds. 28 The blue part

5

30 Intensity (arb. units)

20 10

0 -10

-20

PKE 23.0 - 24.0 eV 20.3 - 20.6 eV 19.4 - 19.7 eV

×0.5

-30 -40 -10

0

10

20 30 Delay time (ps)

40

50

Fig. 4 Integral signals of photoelectron intensities for pyrazine vapor plotted as a function of delay time.

shows two bleached bands corresponding to photoionization from S0 to D0(n-1) and D1(-1). Figure 4 shows the photoelectron signal intensities plotted as a function of delay time. Red circles, green diamonds, and blue triangles (multiplied by a factor of 0.5) indicate the time evolution of S 1, hot S0 state, and ground-state bleach, respectively. The lifetime of S1 and hot S0 is estimated to be 20 ps and more than 100 ps respectively, in reasonable agreement with Horio et al.7 The recovery of ground-state bleach may provide new insight into the dynamics of pyrazine, so further experiments are needed.

3.2 Ring-opening reaction of 1,3-cyclohexadiene

6

Intensity (arb. units)

2

(a)

(a)

1.0 1 0.8

1.2

CHD

1b1g 6ag

-10 ps 2 ps

HT

0.6 0 1

0.4 (b)

HT

0

0.2 -1 0.0

Intensity (arb. units)

1.2

1.0

1b1g 6ag

-10 ps 2 ps

0.8 0.6 0.4 0.2

0.0

CHD

(a)

HT ×50

CHD

-2 8

9

10

11

12

13

Binding energy (eV)

Fig. 5 (a) He(I) photoelectron spectra of CHD and HT 9 convoluted with our energy resolution of 0.5 eV. (b) Difference spectrum of the spectra shown in (a). (c) Photoelectron spectra observed at the delay times of -10 and 2 ps using 4.7 eV pump and 29.5 eV probe pulses. (d) Difference spectrum (Solid, magnified by a factor of 50) and the photoelectron spectrum at -10 ps (dashed) in the present experiment.

The isomerization reaction of 1,3-cyclohexadiene (CHD) to 1,3,5-hexatriene (HT) is a prototypical photochemical ring-opening reaction and has been the focus of many experimental and theoretical studies over the years. 9, 29-39 Photoexcitation places a nuclear wave packet on a steeply repulsive part of the 1B potential energy surface, inducing ultrafast internal conversion ultimately to the 1A ground state via the conical intersection(s).34 Previous studies have shown that the time scale of internal conversion is sub-picosecond.29 As the system returns to the ground state potential, a part of the wave packet returns to the reactant CHD structure and the other to a ring-opening reaction to form HT. There was a controversy about the branching into CHD and HT after the internal conversion to the ground state, and Adachi et al. employed TRPES using the ninth harmonic of the Ti:sapphire laser to estimate the photoisomerization yield to be 30 % and the reaction time to be less than 500 fs. 9 Figure 5(a) shows previously reported He(I) photoelectron spectra of CHD and HT,9 which have been modified to consider the energy resolution (~0.5 eV) in the present experiment. They exhibit almost the same binding energy of HOMO, while those of the second HOMO are different. The difference of the electron binding energy between CHD and HT (Fig. 5 (b)) have been reproduced by quantum chemical calculations by Taketsugu and colleagues. 37 Figure 5 (c) shows the photoelectron spectra observed in this study at the delay times of -10 and 2 ps using 4.7 eV pump and 29.5 eV probe photons, which are to be compared with

7

those shown in ref. 9. Each spectrum has been smoothed to see the fine structure in difference spectrum. The two spectra seem almost identical at a first glance; however, one can clearly identify positive and negative signals by subtracting the spectrum at -10 ps from that at 2 ps as shown in Fig. 5 (d). These features are in excellent agreement with the previous study by Adachi et al. using 14 eV probe pulses. The positive and negative signal intensities remained constant within our experimental time window up to 20 ps, as shown in Fig. 6. Red circles and blue

Intensity (arb. units)

1.0 0.5 Binding energy 9.8 - 10.5 eV 10.5 - 11.8 eV

0.0 -0.5 -1.0 -1.5 -10

-5

0

5

10

15

20

Delay time (ps)

Fig. 6 Integral signals of photoelectron intensities for 1,3-cyclohexadiene vapor plotted as a function of delay time. triangles indicate HT production and CHD depletion, respectively. Sekikawa and colleagues have studied the photochemistry of CHD using their high harmonics light source of 29.5 eV; however, they excited CHD with two-photon absorption of 400 nm (3.1 eV) light to the 3p Rydberg state, so that the subsequent dynamics cannot be compared with those from the valence 1B state. 37 Penberton, Schalk and their coworkers have performed TRPES on the 1B state using the UV pump pulses near 266 nm and the visible – near UV probe pulses.36, 38 While these studies provided the lifetime of the initially excited 1B state, the information for the branching into CHD and HT has hardly been obtained. Because CHD and HT have the same mass, their unambiguous distinction requires spectroscopic observation; TRPES using VUV pulse is useful for studying isomerization reactions. Detailed discussion on the reaction mechanism of CHD is not the aim of this paper, and it is interesting to revisit this reaction with higher time-resolution for elucidating the vibrational wave packet dynamics in the 1B state. However, our results clearly demonstrate feasibility and great potential of TRPES using an HHG light source.

4. Conclusion

8

In summary, a one-box femtosecond laser combined with Kr gas cell and multi-layer mirror provides a simple and convenient setup for TRPES of organic photochemistry. Each experimental data set obtained for pyrazine and 1,3- cyclohexadiene was acquired in several hours, despite rather low probe pulse energies with the order of ten pJ. The rapid data acquisition made possible by 1 kHz high harmonic light source expands opportunities of TRPES. TRPES of liquid microjet using HHG is in progress in our laboratory and will be presented elsewhere.

Acknowledgments This work is supported by JSPS KAKENHI Grant Number 15H05753. C. West is supported by a Research Fellowship for P16036 awarded by the Japan Society for the Promotion of Science.

References 1.

T. Suzuki, International Reviews in Physical Chemistry 31 (2), 265-318 (2012).

2.

T. Suzuki, Bulletin of the Chemical Society of Japan 87 (3), 341-354 (2014).

3.

S. Liu, Y. Ogi, T. Fuji, K. Nishizawa, T. Horio, T. Mizuno, H. Kohguchi, M. Nagasono, T.

Togashi and K. Tono, Physical Review A 81 (3), 031403 (2010). 4.

T. Horio, R. Spesyvtsev and T. Suzuki, Optics express 21 (19), 22423-22428 (2013).

5.

T. Horio, R. Spesyvtsev and T. Suzuki, Optics letters 39 (20), 6021-6024 (2014).

6.

S. Adachi, T. Horio and T. Suzuki, Optics letters 37 (11), 2118-2120 (2012).

7.

T. Horio, R. Spesyvtsev, K. Nagashima, R. A. Ingle, Y.-i. Suzuki and T. Suzuki, The

Journal of Chemical Physics 145 (4), 044306 (2016). 8.

T. Horio, Y.-i. Suzuki and T. Suzuki, The Journal of Chemical Physics 145 (4), 044307

(2016). 9.

S. Adachi, M. Sato and T. Suzuki, The journal of physical chemistry letters 6 (3), 343-346

(2015). 10.

A. McPherson, G. Gibson, H. Jara, U. Johann, T. S. Luk, I. McIntyre, K. Boyer and C. K.

Rhodes, JOSA B 4 (4), 595-601 (1987). 11.

A. L'Huillier, K. J. Schafer and K. C. Kulander, Journal of Physics B: Atomic, Molecular

and Optical Physics 24 (15), 3315 (1991). 12.

P. B. Corkum, Physical Review Letters 71 (13), 1994 (1993).

13.

T. Brabec and F. Krausz, Reviews of Modern Physics 72 (2), 545 (2000).

14.

M. Vrakking, Attosecond and XUV Physics: Ultrafast Dynamics and Spectroscopy. (Wiley

Online Library, 2014). 15.

H. Igarashi, A. Makida, M. Ito and T. Sekikawa, Optics express 20 (4), 3725-3732 (2012).

16.

B. Frietsch, R. Carley, K. Döbrich, C. Gahl, M. Teichmann, O. Schwarzkopf, P. Wernet and

9

M. Weinelt, Review of Scientific Instruments 84 (7), 075106 (2013). 17.

L. Poletto, P. Miotti, F. Frassetto, C. Spezzani, C. Grazioli, M. Coreno, B. Ressel, D.

Gauthier, R. Ivanov, A. Ciavardini, M. de Simone, S. Stagira and G. De Ninno, Appl Opt 53 (26), 5879-5888 (2014). 18.

S. Plogmaker, J. A. Terschlüsen, N. Krebs, M. Svanqvist, J. Forsberg, U. B. Cappel, J.-E.

Rubensson, H. Siegbahn and J. Söderström, Review of Scientific Instruments 86 (12), 123107 (2015). 19.

J. Ojeda, C. Arrell, J. Grilj, F. Frassetto, L. Mewes, H. Zhang, F. Van Mourik, L. Poletto

and M. Chergui, Structural Dynamics 3 (2), 023602 (2016). 20.

H. Mashiko, A. Suda and K. Midorikawa, Optics letters 29 (16), 1927-1929 (2004).

21.

M. Bauer, Journal of Physics D: Applied Physics 38 (16), R253 (2005).

22.

T. Sekikawa, T. Okamoto, E. Haraguchi, M. Yamashita and T. Nakajima, Optics express 16

(26), 21922-21929 (2008). 23.

H. Dachraoui, M. Michelswirth, P. Siffalovic, P. Bartz, C. Schäfer, B. Schnatwinkel, J.

Mattay, W. Pfeiffer, M. Drescher and U. Heinzmann, Physical review letters 106 (10), 107401 (2011). 24.

J. Nishitani, C. W. West and T. Suzuki, Structural Dynamics 4 (4), 044014 (2017).

25.

D. Holland, A. Potts, L. Karlsson, M. Stener and P. Decleva, Chemical Physics 390 (1),

25-35 (2011). 26.

M. Oku, Y. Hou, X. Xing, B. Reed, H. Xu, C. Chang, C.-Y. Ng, K. Nishizawa, K. Ohshimo

and T. Suzuki, The Journal of Physical Chemistry A 112 (11), 2293-2310 (2008). 27.

Y.-I. Suzuki, T. Fuji, T. Horio and T. Suzuki, The Journal of chemical physics 132 (17),

174302 (2010). 28.

E. T. Sevy, M. A. Muyskens, S. M. Rubin, G. W. Flynn and J. T. Muckerman, The Journal

of Chemical Physics 112 (13), 5829-5843 (2000). 29.

W. Fuß, W. Schmid and S. Trushin, The Journal of Chemical Physics 112 (19), 8347-8362

(2000). 30.

M. Garavelli, C. Page, P. Celani, M. Olivucci, W. Schmid, S. Trushin and W. Fuss, The

Journal of Physical Chemistry A 105 (18), 4458-4469 (2001). 31.

N. Kuthirummal, F. M. Rudakov, C. L. Evans and P. M. Weber, The Journal of chemical

physics 125 (13), 133307 (2006). 32.

H. Tamura, S. Nanbu, T. Ishida and H. Nakamura, The Journal of chemical physics 124 (8),

084313 (2006). 33.

F. Rudakov and P. M. Weber, Chemical Physics Letters 470 (4), 187-190 (2009).

34.

S. Deb and P. M. Weber, Annual review of physical chemistry 62, 19-39 (2011).

35.

E. Tapavicza, G. D. Bellchambers, J. C. Vincent and F. Furche, Physical Chemistry

Chemical Physics 15 (42), 18336-18348 (2013).

10

36.

C. C. Pemberton, Y. Zhang, K. Saita, A. Kirrander and P. M. Weber, The Journal of

Physical Chemistry A 119 (33), 8832-8845 (2015). 37.

R. Iikubo, T. Sekikawa, Y. Harabuchi and T. Taketsugu, Faraday Discussions 194, 147-160

(2016). 38.

O. Schalk, T. Geng, T. Thompson, N. Baluyot, R. D. Thomas, E. Tapavicza and T. Hansson,

The Journal of Physical Chemistry A 120 (15), 2320-2329 (2016). 39.

A. R. Attar, A. Bhattacherjee, C. Pemmaraju, K. Schnorr, K. D. Closser, D. Prendergast

and S. R. Leone, Science 356 (6333), 54-59 (2017).

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ω

PE Spectrometer

Kr

19ω

Δt

3ω

e−

Highlights

Time-resolved photoelectron spectroscopy of gaseous polyatomic molecules using 266-nm (4.7 eV) pump and 42-nm (29.5 eV) probe pulses is presented. A 1-kHz Ti:sapphire laser with a 35 fs pulse duration is employed to generate high harmonics in Kr gas, and the 19th harmonic (42-nm) was selected using two SiC/Mg mirrors. Clear observation of the ultrafast electronic dephasing in pyrazine and photoisomerization of 1,3-cyclohexadiene demonstrates the feasibility of TRPES with the UV pump and VUV probe pulses under weak excitation conditions in the perturbation regime.

12