Femtosecond Time-Resolved Photoelectron Imaging - American

Chapter 16

Femtosecond Time-Resolved Photoelectron Imaging 1

Toshinori Suzuki , Li Wang, and Hiroshi Kohguchi

Downloaded by TUFTS UNIV on November 24, 2015 | http://pubs.acs.org Publication Date: October 18, 2000 | doi: 10.1021/bk-2001-0770.ch016

Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan

Femtosecond time-resolved photoelectron imaging (FS-PEI) was applied to ultrafast dephasing processes in pyrazine. For S state, known as the best example for intermediate case in radiationless transition, FSPEI visualized the decay of optically-prepared singlet character in 100 ps and corresponding build-up of triplet character, for the first time. Photoexcitation to the S state exhibited dephasing to S within our time -resolution(400 fs) and subsequent decay to S in 20 ps. 1

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1

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Introduction Molecular photodissociation dynamics has been studied extensively by measuring quantum state and scattering distributions of products. ' However, undoubtedly, the most unambiguous experimental elucidation of reaction mechanism is by real-time observation of the reaction. The best example for it is femtosecond pump-probe fluorescence spectroscopy on non-adiabatic dissociation dynamics of Nal by Zewail and coworkers. ' In a pump-probe experiment, a probe pulse projects the wave packet motion on an excited state surface to a higher electronic state. Therefore, the prerequisite for this approach is precise knowledge on this higher state. However, most of higher excited states have complicated surface crossings and spectroscopic data on these states are very limited. Considering this difficulty in pump-probe experiments between neutral states, photoelectron spectrocopy " seems more advantageous, since it projects the dynamics onto a cation potential that is accurately characterized by spectroscopy and/or computationally tractable. Other advantages of photoelectron spectroscopy are (1) capability of detecting both singlet and triplet states, enabling direct observation of intersystem crossing and internal conversion processes, (2) high sensitivity due to efficient collection of electrons by electromagnetic fields, and (3) applicability of ultrafast laser with fixed wavelength. As with Raman spectroscopy, a laser wavelength can be fixed while electron energies are dispersed to observe vibrational dynamics. 1 2

3 4

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9

Corresponding author (email: [email protected]). © 2001 American Chemical Society

In Imaging in Chemical Dynamics; Suits, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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As for the analysis of photoelectron scattering distribution, the time-of-flight (TOF) method has been widely with pulsed light sources (Table I). However, a standard TOF method has a poor collection efficiency of electrons, which are also highly susceptible to stray fields. The magnetic bottle spectrometer developed by Kruit and Read significantly improved the efficiency, but angular resolution was sacrificed. Photoelecton imaging (PEI) pioneered by Helm et al., on the other hand, achieves the highest collection efficiency of electrons while providing routine measurements of speed and angular distributions of photoelectrons. This work combines PEI with femtosecond pump-probe technique, for the first time, and examines its performance through observation of ultrafast dephasing in an isolated molecule.


10

u

Figure 1. Photoelectron spectroscopy of wave-packet motion. 1213

1

Since the pioneering work by Frad, Lahmani, Tramer and T r i e , the S ^ ! ^ ) state of pyrazine has been the best-known example of an intermediate case in molecular radiationless transition. ' It was predicted that coherent excitation of an intermediate case molecule exhibits biexponential fluorescence decay I(t) 12 15

13

9

where the fast decay is the ultrafast dephasing of an optically-prepared singlet state into the mixed singlet-triplet character (the first term of the above formula) and the slow decay is the depopulation of this mixed state (the second term). 1213

Table I. Comparison of Photoelectron Spectroscopic Methods Method Electrostatic TOF Magnetic bottle Imaging

Resolution (meV@leV) >5

Acceptance Angle (steradian)

lO'Mo

3

CW light

Angular Dist. Measurement Inefficient

OK

>3 >10

Inefficient

No

2π

Not Possible

No

>20

4π

Routine

OK


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The biexponential fluorescence decay of S i pyrazine was extensively studied in the 1980's, and lengthy debate ensured as to whether the fast component experimentally observed was due to dephasing predicted or Rayleigh-Raman scattering. With the development of picosecond laser spectroscopy, convincing evidences for dephasing (x~100ps) have been obtained, ' and the consistency was also found with molecular eigenstate spectroscopy pioneered by Kommandeur, Meerts and coworkers. However, it is noted that these works only observed time-evolution of the singlet character of the excited state, |(s|ip(0)| > * the dynamics in the triplet manifold, 16 21

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a n c

J (l I yKO)| has not been observed. The present work revisits this classic problem by Downloaded by TUFTS UNIV on November 24, 2015 | http://pubs.acs.org Publication Date: October 18, 2000 | doi: 10.1021/bk-2001-0770.ch016

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femtosecond time-resolved photoelectron imaging and sheds light on the dark triplet manifold. Internal conversion from S state of pyrazine has been considered in a pioneering theoretical work on femtosecond photoelectron spectroscopy by Seel and Domke. Although our experimental time-resolution (450 fs) is much lower than the estimated electronic dephasing time of 30 fs, we applied FS-PEI to this problem and attempted to extract dynamical information. 2

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Experimental

A. [1+2'] ionization of pyrazine via Si state A solid-state femtosecond laser system consists of a diode-pumped Ti:sapphire oscillator, 10Hz YAG-pumped Tirsapphire regenerative amplifier, and optical parametric amplifier. Tunable U V light (λ< 323 nm) and the second harmonic (396 nm) of Tirsapphire fundamental were optically delayed and irradiated onto the molecular beam. The cross correlation of the pump-probe pulses was about 400 fs. The pump pulse excites pyrazine in a molecular beam 0.3-3% seeded in He (a stagnation pressure 1 atm to the vacuum) up to the Si 0° level and a probe pulse further ionizes them. The photoelectrons are accelerated by an electric field parallel to the molecular beam and projected onto a position-sensitive imaging detector. The acceleration field provides two-dimensional space focussing, so that the image only reflects the linear momentum of the electron parallel to the detector face. The fieldfree region (44 cm) of the TOF spectrometer was shielded with a μ-metal tube to avoid external magnetic fields that might otherwise deflect the electron trajectories. The imaging detector consists of a microchannel plate (MCP), a phosphor screen, and a charge-coupled device (CCD) camera. The video signal (25 frames/sec) is transferred to a computer on which a real-time image processing calculates the center of gravity of each light spot to enhance imaging resolution and counts the number of electrons arrived at each pixel over a number of laser shots. The electron images thus 25


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obtained were inverted to electron speed-angular distribution by inverse Abel transforms.

Figure 2. Schematic diagram of photoelectron imaging apparatus. The probe wavelength of 396 nm energetically allows two-photon ionization from both the singlet and triplet states (IP=74 908 cm" ). The power density of our pump laser at the interaction region was less than 10 W/cm , while that of the probe laser was about 5 x l O W/cm . For these power densities, ponderomotive shift of the photoelectron kinetic energies and the alignment of ground-state molecule in the laser field can be neglected. We also measured the S i « - S spectrum of pyrazine in the molecular beam by a nanosecond laser and ascertained good rovibrational cooling and negligible clustering. The bandwidth of our pump pulse (110 cm' ) does not allow selection of rotational lines but excludes excitation to other vibronic levels than 0°. 1

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2

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1

probe 396 nm dephasing Si(n,?i*)

405ST Τι(η,π*) pump 323 nml

Figure 3. [1+2 '] REMPI of pyrazine.


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Β. [1+1'] ionization of pyrazine via S state 2

The third harmonic (262 nm) of Tirsapphire laser was used to excited pyrazine to S state and 220 nm light ionized them. The probe pulse was generated by non-linear mixing of the U V light from ΟΡΑ with the Tksapphire fundamental. A l l other experimental procedures are the same with the Si case.


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Results and Discussion

A . Si State of Pyrazine

(a)

τ =98 ± 4

( ) c

0.54

100

200

300

400 500

Figure 4. (a) Time dependence of total photoelectron signal in femtosecond [1+2 '] REMPI ofpyrazine via the SjfBsufaK*)] 0° level The pump and probe wavelengths are 323 and 396 nm, respectively, (b) Photoelectron signal for the kinetic energy Ε > 630 meV. (c) Photoelectorn signal for the kinetic energy E< 630 meV.

Time delay /ps In Imaging in Chemical Dynamics; Suits, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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Figure 4 shows the photoelectron intensity observed as a function of time delay in [1+2'] ionization of pyrazine via Si State. As seen in Fig. 4(a), the total electron current (= integral photoionization cross section) shows no time-dependence, which apparently contradicts the fast fluorescence decay data reported previously. " Note, however, that photoionization can occur both from the singlet and triplet manifolds. Therefore, Fig. 4(a) simply implies that population decay from the mixed singlettriplet states does not occur in this time range. The depopulation of the mixed states takes place in 300-400 ns. Figure 5 shows the snapshots of photoelectron speed and angular distributions. Inspection of these images immediately reveals the ionization from the singlet and triplet characters as the outer and inner rings, respectively. Ionization from the triplet results in low photoelectron energy, because the triplet states isoenergetic to the singlet have large vibrational energies (4055 cm" in the case of T O , and FranckCondon overlap favors ionization to highly vibrationally-excited states in the cation. (The bright spot in the middle of the image is due to the low energy tail of the inner ring. Photoelectrons with near zero kinetic energy are concentrated in the center of the image with a weighting factor that scales with the kinetic energy of electron or the image radius squared.) The singlet character decays in less than 200 ps, and the triplet character builds up correspondingly. The selective measurements of time-dependent singlet and triplet characters were performed by observing light spots in the outer and inner parts of images on the phosphor with masks and a photomultiplier tube. The results are shown in Fig. 4(b) and 4(c). The high energy electron decayed with τ = 108±2 ps, in excellent agreement with fast fluorescence decay observed previously, while the low energy electron increased correspondingly with τ = 98±4 ps (Fig. 4(c)). 16


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Figure 5. Inverse Abel transforms (512 x 512 pixels) of photoelectron images of [1+2 'J REMPI of pyrazine via the Si[ B (n,n*)] 0° level at the time delays of (a) 0.3, (b) 30, (c) 200, and (d) 500 ps. The original images were integrated for 36 000 laser shots. l

3u


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The result clearly demonstrates that the coherent decay of singlet character is compensated by the build-up of the triplet character; S (t)=-L (t). This is because the decay from the excited state manifold to the ground state is negligible in this time range. Similar experiment on pyrazine-d revealed essentially the same behavior with slightly shorter dephasing lifetime, τ = 80 ps. El-Sayed has speculated that efficient intersystem crossing in pyrazine ( O i s c l ) is mediated by Τ (π,π*), since direct spin-orbit coupling is not allowed for Si(n,7i*) Τ^η,π*) but allowed for 8ι(η,π*)-Τ (π,π*). This idea has been widely accepted by researchers, although it has not been fully proved. Quantum mechanical electronic structure calculations have been performed on pyrazine, ' but the energies of (η,π*) states in diazine are strongly influenced by through-bond interaction between the two nitrogen atoms. ' Ab initio calculations considering extensive configuration interaction including σ-orbitals are required to answer this question. Careful inspection of the snapshots reveals that the ring due to triplet states shrinks in time; the time delay of 30 ps is a critical point where the two rings in the triplet part are equal in intensity. This suggests the possibility that relaxation occurs in the triplet manifold. The time evolution of the system can be examined more quantitatively in photoelectron kinetic energy distributions presented in Fig. 6. The feature at 160 meV appears instantaneously with the light pulse, but it is overtaken by the growth of another peak at 100 meV. The peak at 100 meV is assigned to ionization from Τ^η,π*). If the peak at 160 meV is assigned to ionization from Τ (π,π*), the time evolution of Si(n,7i*), Τ (π,π*), and Τ^η,π*) provides experimental evidence for intersystem crossing mediated by Τ (π,π*) suggested by El-Sayed. 13

coh

coh

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=

2

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2

30 31


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2

L0

Kinetic energy /meV Figure 6. Photoelectron kinetic energy distributions in femtosecond [1+2'J REMPI of pyrazine via the Srf'BsyfaTC*)] 0° level at the time delays of0.3, 30, 200, and 500ps.


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However, assignment of the peak at 160 meV to a higher excited (Rydberg) state resonant by Av(323 nm)+ Av(396 nm) can not be excluded. We are performing [1+Γ] photoelectron imaging to examine it further. The angular anisotropy of the photoelectron distribution slightly varied with time delay, but the β values at 60 ps were 1.1 and 1.7 for the singlet and triplet channels, respectively. As Piancastelli et al. have shown for one-photon ionization of pyrazine from the ground state, the anisotropy is expected to change with photoelectron energy, especially for the ejection of π electrons. Therefore, the interpretation of the anisotropy observed at one ionization wavelength is not possible at the point. Finally, although our experiment employed a static electric field (< 700 V/cm) to extract photoelectrons toward the detector, the Stark effect on the short time dynamics of pyrazine can be excluded. Previous works have shown that noticeable Stark shift of the level structure is only induced by an order of magnitude higher field strength (> 10kV/cm). ' The dephasing rate we obtained is in excellent agreement with the previous fluorescence decay data under field free conditions. '


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B. S state of Pyrazine 2

The second excited singlet state of pyrazine S [ B ( K , K * ) ] is located 7 0 0 0 cm" above the S ^ r ^ O v i * ) ] state. The S (7i,7t*)Sj electronic dephasing. Seel and Domcke have reported the theory of femtosecond time-resolved photoelectron spectroscopy for this dephasing process. We excited pyrazine in a molecular beam with 2 6 2 nm light to the vicinity of the zero vibrational level in S (ft,7t*) and subsequently ionized the excited molecules with 2 2 0 nm light. As shown in Fig. 7, the total photoelectron (and parent mass) signal decayed with the lifetime of 2 2 ± 1 ps. Similar measurement on pyrazine-d4 showed the lifetime of 3 9 ± 1 ps. 2

2

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2u

0

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Figure 7. The decay of total photoelectron intensity in [1+Γ] REMPI via S state of (a) pyrazine and (b) deuteratedpyrazine. 2

0

50

100

Time delay /ps


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The observed lifetime for pyrazine-h4 is in agreement with the lifetime, < 50 ps, of the higher vibronic level of Si studied by Yamazaki et al, from which we assign the observed decay to the electronic relaxation from the Si manifold after ultrafast dephasing from S , although the latter is undetectable with our apparatus. The intersystem crossing yield from Si pyrazine is known to sharply fall off near 280 nm, so the observed decay is ascribed to Si —>S internal conversion. The longer lifetime observed for pyrazine-d4 is ascribed to the reduction of the Franck-Condon overlap for the accepting modes between Si and S . 18

2

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0


0

Figure 8. Instantaneous formation ofS) from S and its subsequent decay to S . 2

0

Conclusion and Outlook The present work demonstrated high performance of time-resolved photoelectron imaging for the study of excited state dynamics. The two-dimensional position sensitive detection allows visualization of speed-angular distribution of photoelectrons routinely, and the pump-probe technique provides snap-shots of the distribution as a function of time. The energy resolution is enhanced by center-of-gravity calculation for light spots captured by a C C D camera, and thresholding/counting method corrects non-uniform sensitivity of the position sensitive detector. The energy resolution ΔΕ/Ε=0.05 is


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easily achievable with this method. Modification of an electrode design will provide ΔΕ/Ε

Femtosecond Time-Resolved Photoelectron Imaging - American

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