Anisotropy in Time-Resolved Photoelectron Spectroscopy in the Gas

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Anisotropy in Time-Resolved Photoelectron Spectroscopy in the Gas Phase Oliver Schalk, and Andrey E. Boguslavskiy J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b10490 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on December 3, 2017

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The Journal of Physical Chemistry

Anisotropy in Time-Resolved Photoelectron Spectroscopy in the Gas Phase Oliver Schalk, 1

1,2,*

1,3

Andrey E. Boguslavskiy

National Research Council Canada, 100 Sussex Drive, Ottawa, Ontario K1A 0R6, Canada 2

Department of Physics, AlbaNova University Centre, Stockholm University, Roslagstullsbacken 21, 106 91 Stockholm, Sweden

3

Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, Ontario K1N 6N5, Canada * electronic address: [email protected]

Abstract Transient absorption anisotropy is a well-established technique in time-resolved liquid phase spectroscopy. Here, we show how the technique is applied in the gas phase for time-resolved photoelectron spectroscopy and what type of additional information can be obtained as compared to other techniques. We exemplify its use by presenting results on rotational revivals in pyrazine after excitation at 324 nm and provide new insights into two recent experiments: (i) the difference between Rydberg and valence state excitation after one and two photon absorption in butadiene, and (ii) excitation to the two lowest lying vibronic modes of the degenerate π3p Rydberg state in 1-azabicyclo[2.2.0]octane. Going forward, we expect the technique to be used on a regular basis, especially with the advent of high harmonic probe sources and liquid beam setups where other techniques to extract polarization dependent information such as velocity map imaging cannot easily be applied.

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1 INTRODUCTION Transient anisotropy is a widely used technique in classical liquid phase pump-probe spectroscopy.1,2 In this technique, a molecular sample is excited by a linearly polarized laser pulse and the changes induced in the molecules are recorded with a time-delayed probe pulse (e) being polarized either parallel (I||) or perpendicular (I⊥) with respect to the pump pulse (p). The anisotropy parameter is then obtained through r (t,Ep,Ee) = (I|| (t,Ep,Ee) - I⊥ (t,Ep,Ee)) / (I|| (t,Ep,Ee) + 2 I⊥ (t,Ep,Ee)),

(1)

where t is the time delay between the pump and the probe pulse, and Ep and Ee are the energies of the pump and the probe photon. Observables are usually the change of optical density or an up-converted fluorescence signal. Typically, one can consider a pump-probe experiment in the following way: First, the excitation pulse generates a cos2nθ-distribution along its polarization axis with respect to the transition dipole moment of the molecules, where n is the number of pump photon used for the excitation.3 Afterwards, the probe pulse ionizes the pre-oriented sample according to the polarization dependent ionization cross section. Several processes can be observed with liquid phase transient anisotropy. Prominent examples are: (i) The rotational behavior of molecules including detection of diffusional reorientation, which often takes place on a ps-ns time scale.2 The initial anisotropy signal has Gaussian shape4 and depends only on the rotational constants in low density or gaseous samples.5,6 (ii) Internal vibrational relaxation in molecules with degenerate modes where the transition dipole moments rapidly redistribute.7 (iii) Intramolecular Förster resonance energy transfer of biomolecules, which can be detected by fluorescence anisotropy because the donor and the acceptor part of the molecule exhibit different orientations of the fluorescence dipole moment.8 (iv) Electronic dephasing of molecules that are excited to degenerate excited states such as porphyrins and other high symmetry transition metal complexes9,10 (v) Spectroscopically dark states that are accessible by the probe pulse in a pump-probe scheme. 11

(vi) Pseudo-degenerate states as given through the mixing of two excited states in the

Franck-Condon region of a molecule.12 Equation (1) can also be used in photoelectron spectroscopy.13-16 Then, I|| (t,E) and I⊥ (t,E) present electron counts depending on the polarization and the time delay t of the probe pulse with respect to the pump pulse and the photoelectron kinetic energy (PKE). At a first glance, the interpretation of the results is not as straightforward as for transient absorption


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experiments because the outgoing photoelectron carries information in form of an angular momentum.17-18 However, the intensities I|| (t,E) and I⊥ (t,E) are only defined by the cross section, and consequently, the anisotropy is independent from β-parameters.14 Therefore, interpretation of these data promises to be more straightforward than of the information obtained by, e.g., velocity map imaging (VMI).19 Moreover, the experimental setup does not require an additional alignment pulse; it allows measurements in liquid jets16 and also using high photon energies which causes problems for VMI. This will be detailed in the discussion section. In the following, we are going to examine the information that can be obtained with gas phase time-resolved photoelectron anisotropy (TRPEA) on a couple of benchmark molecules. In section III.A, we exhibit experiments on rotational rephrasing and compare the results with VMI experiments under identical conditions. In section III.B, we show how information on the symmetry of excited states can be extracted from TRPEA experiments and how different electronic bands can thus be distinguished. As an example, we use two photon excitation at 400 nm in buta-1,3-diene which accesses two different excited states. Section III.C demonstrates how details on Rydberg state dynamics can be obtained in the case of 1azabicyclo[2.2.2]octane (ABCO). Finally, we give an outlook on the capabilities of the technique in section III.D.

II EXPERIMENTAL SETUP Our TRPES setup including laser system and 4π-magnetic bottle spectrometer has been described earlier.20 Tunable pulses were generated using two TOPAS (Fa. Light conversion) and subsequent frequency mixing and doubling schemes. Butadiene (Matheson) with a nominal purity of >99% was diluted in helium (1:99 mixture) and expanded into vacuum using an Even−Lavie valve with a conical nozzle of 200 µm diameter. Pyrazine and ABCO were purchased from Sigma-Aldrich with nominal purities of 99 % and 97 %, respectively, and brought into the gas phase through an Even-Lavie valve with a backing pressure of 3 bar helium. The expected rotational temperature of the molecules under the experimental conditions was estimated before to 50-100 K15,21. Energy calibration was performed using NO. This molecule also served to determine the in situ cross correlations, which were 140 fs for the pyrazine and ABCO experiments and 120 fs for the butadiene experiment. Set-up and calibration of the polarization scheme were explained before in Ref. 15. Here, we provide some additional information about the calibration procedure. The polarization of the



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laser pulses was controlled by two Berek compensators (New Focus). Typically, we recombine pump and probe beam on a dielectric mirror where the pump pulse is reflected and the probe pulse is transmitted from the back. After the recombination mirror, we use aluminum mirrors in order to not influence the properties of the different polarization directions. During experiments, we fix the polarization of the probe pulses and rotate the polarization of the pump pulses only. First, we checked the dependence of the one color signal from the polarization angle χ with respect to the axis of the magnetic bottle spectrometer using two photon ionization of buta1,3-diene at 263 nm. A typical trace is shown in Figure 1. The fitting was done with a double cosine function: I(χ) = I0 + Iχ cos (χ) + I2χ cos (2χ)

(2)

The count rate I shows a strong dependence on χ which can have different reasons: (i) anisotropic collection efficiency of the spectrometer, (ii) orientation of the sample by the expansion into the vacuum, (iii) a spatial offset of the beam by the optics behind the Berek compensator, and (iv) different reflection properties for s and p polarized light of the UV enhanced aluminum mirrors. We ruled out (iii) by slightly modifying the beam path, which resulted in the same calibration curve just with a different absolute count rate and note that (iv) is likely the most important reason. In order to avoid artifacts, the experiments are best operated under conditions, where the two polarizations give the same electron count rates when separated by 90°. In addition, we wanted to operate at 45° angle with respect to the magnetic bottle to avoid any unwanted effects from pre-alignment of the sample (ii). Therefore, positions (a) in Figure 1 were discarded, because the angles to observe the same electron counts were 50° and 140° with respect to the magnetic bottle spectrometer. Instead, we used positions (b) (angles 135° and 225°). In order to test the stability of the anisotropy signals, we performed experiments at different days with individual calibrations. Typically, the values had a fluctuation of the anisotropy of δr = ± 0.03. During experiments, we recorded five different spectra at each delay time: Pump-probe signal at parallel and perpendicular polarizations, pump-only signal at parallel and perpendicular polarizations and one probe-only signal since the pump beam was not rotated during experiments.


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III RESULTS AND DISCUSSION III.A Rotational revivals in pyrazine In order to test the capabilities of TRPEA, we first studied rotational rephasing of pyrazine upon excitation at 324 nm and two photon probe at 401 nm, mirroring the conditions of the VMI experiments from Suzuki and coworkers.22 Excitation promotes the molecule to the S1 state (1B3u symmetry, nπ* character), from where it undergoes intersystem crossing to the T1 state within 105 ± 10 ps.22 The full photoelectron spectrum measured under magic angle conditions is depicted in Figure 2. The one-photon pump, two-photon probe [1,2’] energy cutoff is at a PKE of 0.72 eV assuming an ionization potential of 9.29 eV.23 The probe pulses ionize the sample from the S1-state either directly or via a p-Rydberg state around a PKE of 0.58 eV (slightly shifted toward lower kinetic energies as compared to the work of T. Suzuki) or an s-Rydberg state at a PKE just above 0 eV. In addition, we observed the raise of the triplet state via three photon probe at 3.0 eV. Our parallel and perpendicular signals as well as the retrieved anisotropies for selected PKE of the S1-band are shown in Figure 3. We clearly observe the expected Gaussian decay of the anisotropy at early times, which is caused by rotational dephasing,24 and the rotational revivals generated by the photoselective pump pulse4 with the same revival times of ~ 41 ps and 82 ps as reported earlier for pyrazine.22 The revivals have a more pronounced signature on the perpendicular component which is typical for anisotropy experiment.3 The anisotropy signal obtained from ionization through the pRydberg state is similar to the alignment parameter A20/A00 in Ref. 22. However, the anisotropy signal obtained from ionization through the s-Rydberg state just above 0 eV PKE has opposite sign with respect to the alignment parameter but is similar to ionization through the p-Rydberg state. This does not astonish as both experiments record different parameters. This is discussed in more depth in the next section. The anisotropies at the positions where no revival is observed vary slightly between r ≈ 0.05 for ionization via the p-Rydberg state and r ≈ 0.03 for ionization via the s-Rydberg state. These are typical values for anisotropy experiments on a dephased gas phase sample when pump and probe transition dipole moments are not oriented perpendicular to each other.3 In addition to the revivals in the [1,2’] ionization region, we also observe revival structures in the [1,3’] region, most notably from the triplet state at 3.0 eV. This signal is added to Figure 3. It is noisy because of the low count rate but the revival signature is clearly visible. It appears at a slightly shorter delay time which might be due to different geometries in the singlet and the triplet state which causes a different moment of inertia in the triplet state. ACS Paragon Plus Environment


III.B Ultrafast dynamics of butadiene upon one and two photon excitation The second example to highlight the usefulness of TRPEA is the dynamics induced by two photons of 400 nm in butadiene. The time-resolved photoelectron spectrum was investigated before and it was suggested that excitation populates both, the π3s Rydberg state and the twophoton allowed 2Ag state.25 In Figures 4 a and b, the photoelectron spectrum for parallel pump and probe polarizations as well as the TRPEA spectrum are shown, respectively. When compared to the signal under magic angle condition (see Figure 1 in Ref. 25), one already realizes that the peak that was previously assigned to Rydberg state excitation (around 1.9 eV, the photoelectron spectrum is slightly shifted toward higher energies in our experiments because of the smaller probe wavelength of 262 nm) is significantly weaker as compared to the rest of the spectrum. This behavior is also reflected in the anisotropy signal. For π3s Rydberg state excitation, the anisotropy is close to zero, which is expected for an s-Rydberg state. The fact that the value is higher can be explained by the strong Rydberg-valence coupling which was discussed previously.25 Note that there is a distinct difference between direct ionization from a Rydberg state and ionization through a Rydberg state. For example, ionization from an s-Rydberg state can occur along any spatial direction while ionization through an s-Rydberg state first requires population of the state which, in turn, requires a transition dipole moment that is represented by a vector. As a consequence, the anisotropy is expected to be higher even though the outgoing electron wave, as it would be measured by a VMI experiment, might be identical. The latter was observed in the pyrazine experiments in section III.A, where VMI experiments showed an opposite sign for ionization through the sand the p-Rydberg state22 while the anisotropy signature was almost identical. The peak on the red side (i.e. lower PKE) of the Rydberg band originates from direct excitation to the 1Ag (π*)2 state and shows an anisotropy of ~ 0.55. This is close to the maximum value of 0.571 that can be obtained in transient absortion spectroscopy when two pump photons are used.3 In fact, the π* orbitals in butadiene have a node along the molecular axis from where electron emission is supposed to be zero which explains the highly anisotropic behavior. The signal at lower PKE originates from ionization at geometries that are further down on the potential energy surface. Butadiene is supposed to undergo large amplitude motions which includes out of plane twisting of one terminal CH2-group.26 Upon twisting, the ionization potential rises such that less kinetic energy remains for the photoelectrons.27 This twist also deforms the π-orbital. As a consequence, electron emission


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becomes more evenly distributed which explains the decrease in anisotropy. The increased value of r ≈ 0.7 at PKE > 1.9 eV is a sign for a two photon pump, two photon probe process and agrees with the theoretical value of 0.780 (here, we used less probe intensity and a higher probe photon energy than in ref. 25 such that we do not observe superexcited states caused by absorption of two photons of 267 nm). The dynamics observed in the two photon excitation experiment can be contrasted with the signal upon one photon excitation at 216 nm to the bright 1Bu(ππ*) state shown in Figure 5. The TRPES signal in Figure 5a agrees with the signal observed in previous experimental14-15 and computational work.28 In brief: upon excitation, the molecule is supposed to undergo large amplitude motions.26 which are reflected by a shifting photoelectron signal at lower PKE (see, e.g. ref. 29). The signal can be followed further in the two photon probe region above 1.2 eV. In this region, the signal likely originates from the dark 1Ag(π*)2 state. This can be verified by TRPEA: In the one photon region, the time zero anisotropy is 0.25, slightly dropping toward smaller PKE. In the classical picture, this anisotropy would denote an angle of π/4 between pump and probe transition dipole moment. In the two photon probe region, anisotropy is higher which cannot be explained by the additional probe photon, because the resulting anisotropy should also be 0.25 at an angle of π/4 between pump and probe transition dipole moment.3 Therefore, the signal must originate from a different electronically excited state. In addition, we observe a drop of the anisotropy to ~0.05 in the one photon probe region which is reached within 350 ± 60 fs. This is significantly slower than the dynamics of butadiene but roughly agrees with the rotational dephasing time of butadiene, which is τrot = (J/3kBT)

1/2

= 310 fs for the smallest moment of inertia J30 and an estimated rotational

temperature of T = 50 K in the molecular beam.

III.C Rydberg state dynamics in 1-azabicyclo[2.2.2]octane Our last example is ABCO (see Figure 6 for the structure), on which VMI-measurements have been performed previously.31 Excitation at 228.6 nm promotes the molecule to the vibrational ground state of the S2 state, which has 1E-symmetry and π3p Rydberg character. The newly populated molecular orbitals are mainly centered at the carbon atoms next to the nitrogen atom.32,33 At a probe wavelength of 400 nm, electrons from the S2 state appear at a PKE of 0.8 eV. From there, the molecule undergoes internal conversion to the π3s Rydberg



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state (2A-symmetry) observed at about 0.2 eV PKE. The data can be fitted with two exponential time constants of τ1 = 0.81 ± 0.1 ps and τ2 = 2.3 ± 0.3 ps. The first time constant describes the internal conversion between S2 and S1 and is a bit longer than the 620 ± 80 fs found upon excitation at 201 nm31 while the second time constant mainly describes a shift of the S1 band towards slightly higher PKEs (see below for a discussion). A small amplitude of the τ2 contribution can also be found on the S2 band. In addition, the molecule shows a shift of the spectrum in the S2 band, where the rise of the spectrum is delayed by 60 ± 20 fs at the red edge of the band, which is a sign that the molecule rearranges its structure in the excited state.29,34 This is thought to be caused by a Jahn-Teller splitting and a concomitant deformation.31 Excitation of ABCO at the first excited vibronic band at 223.8 nm does not show any major differences in the time constants, and we extracted values of τ1 = 0.86 ± 0.1 ps and τ2 = 2.0 ± 0.3 ps. Anisotropy traces of ABCO are shown in Figure 6 for both, the S1- and the S2-band as well as for 228.6 and 223.6 nm pump. The anisotropies vary slightly across the bands, however, no obvious trend can be observed. The integrated anisotropies for the whole bands are depicted with black squares. At 228.6 nm pump, the S2-band exhibits a starting value of 0.16 ± 0.01 which decays within τr2 = 0.6 ± 0.3 ps to a final value of 0.04 ± 0.01. The S1 band starts at -0.005 and rises to 0.005 within τr1 = 0.9 ± 0.3 ps. At 223.6 nm, the time constants are the same as for 228.6 nm, but the starting and final values for the anisotropy of the S2 state are lower (decay from 0.11 ± 0.01 to 0.01 ± 0.01) while the ones for the S1 state are higher (rise from 0.02 to 0.04). The time constants agree with the time dependence of the β2- and β4parameters in VMI measurements upon excitation at 201 nm.30 Finally time zero anisotropies can be explained in a similar way to the last section: Since the S2 state has E symmetry, it shows a directionality which leads to a non-zero anisotropy35 while the “spherical” s-Rydberg state leads to an anisotropy close to zero. In ABCO, the valence states are high up in energy and no Rydberg-valence mixing is expected, which leads to lower anisotropy values as compared to butadiene. The easiest explanation for the time dependence of the anisotropy would be rotational dephasing; however, an estimation of the 1/2

dephasing time leads to τrot = (J/3kBT)

= 1.5 ± 0.2 ps for a spherical molecule3 (the

principle moment of inertia of ABCO is J = 3.46 x 10

-45

kg/m2

36

) which is distinctively

longer than the fast decay times. In addition, rotational dephasing shows a Gaussian type decay at early times as observed, e.g., for pyrazine (see Figure 3) and cannot explain the rise of the anisotropy for the S1-band. The main reason for a decrease of anisotropy is a character


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change of the excited states during the course of the reaction as was proposed before.31 The non-zero value and the time dependence of the anisotropy of the S1 band matches the nonzero value of the β4 parameter in VMI measurements in as much as both show that the character of the band is not exclusively s-Rydberg like and that it changes during the course of the experiment due to a decreased mixing with the p-Rydberg states within the first ps. The observed dephasing time of the S2 state is on the same time scale as observed before in other systems with E symmetry in liquid phase (see e.g. Ref 10). The theoretical long time limit of 0.1 is not reached because the contribution of rotational dephasing is non-negligible and because of the differences between transient anisotropy and TRPEA. In addition, the existence of a coupling between the S1 and the S2 state causes a pseudo-rotation of transition dipole moments similar to the coupling observed in diazabicyclo[2.2.2]octane37 Our cross correlation might be too long to see oscillations, but the coherence is a candidate to explain the second exponential component on our TRPES-signal.

III.D A Brief Outlook on the Capabilities of the Technique There are several possibilities to use TRPEA beyond gas phase spectroscopy with near-UV pulses. Here we give an overview of potential applications: (i)

Gas phase experiments with VUV or XUV pulses

When the photon energy of the probe pulse is too high, VMI is not a suitable technique anymore, because the photoelectrons are too fast and cause a bad energy resolution. This is valid for high harmonic generation sources operating slightly above 20 eV and even more so for X-ray beams. Instead, experimentalists rely on magnetic bottle setups which allows for slowing down the photoelectrons. We envisage TRPEA to be a possibility to gain insight into complex bands structures of molecular samples. (ii)

Time-resolved photoelectron spectroscopy in liquid beams

VMI experiments are infeasible in liquid beam experiments. Instead, magnetic bottle spectrometers are also used here.16 Since electron yields only depend on ionization cross sections, TRPEA is a suitable technique. Many papers have been published on liquid phase transient anisotropy [1, 2] and the values that can be extracted might be interpreted in a similar fashion. For example, one can follow dephasing processes, energy transfer or simply differentiate between various excited states. One has to keep in mind that time-resolved photoelectron spectroscopy in liquid beams is a surface dependent technique, though. ACS Paragon Plus Environment


Experiments will be required to see what information content can be obtained. (iii)

Photodissociation dynamics

Our third suggestion are experiments on j-j or j-v coupling following photodissociation of a molecule. Several restrictions apply, for example, that dissociation must be much faster than the reorientation time of the molecules and the probe laser pulse must be able to selectively probe one of the fragments. However, once these criteria are fulfilled, we expect nice experiments using TRPEA. This kind of experiment can be complemented with electron-ion coincidence measurements.

IV. CONCLUSION In conclusion, the three highlighted examples together with the previous demonstrations15,16 show that TRPEA is a technique that can be used to extract valuable information about the investigated molecules. In comparison to VMI measurements, it has been shown that TRPEA is not sensitive to non-zero β-parameters14 which simplifies the interpretation, especially for larger molecules, where knowledge from liquid phase experiment can be used to interpret the data. TRPEA is a technique which we think will be useful in the future. In order to facilitate the analysis, we are currently working on a more comprehensive theoretical description of the observed effects and plan to present further examples.

AUTHOR INFORMATION Corresponding Author * (O.S.) E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Albert Stolow for his generous support and for the possibility to perform the measurements using his equipment and Paul Hockett for many helpful discussions. O.S. thanks the Humboldt foundation for a research fellowship.


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[24] Baskin, J. S.; Zewail A. H. Femtosecond Real-Time Probing of Reactions. 15. TimeDependent Coherent Alignment. J. Chem. Phys. 1994, 98, 3337-3351. [25] Schalk, O.; Boguslavskiy, A. E; Stolow, A. Two-Photon Excited State Dynamics of Dark Valence, Rydberg, and Superexcited States in 1,3-Butadiene. J. Phys. Chem. Lett. 2014, 4, 560-565. [26] Levine, B. G.; Todd J. Martınez, T. J. Ab Initio Multiple Spawning Dynamics of Excited Butadiene: Role of Charge Transfer. J. Phys. Chem. A 2009, 113, 12815–12824. [27] Schalk, O.; Boguslavskiy, A. E.; Stolow, A.; Schuurman, M., Through-Bond Interactions and the Localization of Excited-State Dynamics. J. Am. Chem. Soc. 2011, 133, 1645116458. [28] Levine, B. G. Nonadiabatic Dynamics of cis-trans Photoisomerization - A First Principles Study, PhD Thesis, University of Illinois at Urbana-Champaign, 2007. [29] Wolf, T. J. A.; Kuhlman, T. S.; Schalk, O.; Martinez, T. J.; Møller, K. B.; Stolow, A.; Unterreiner, A.-N. Hexamethylcyclopentadiene: A Test Case for the Combination of Time Resolved Photoelectron Spectroscopy and ab initio Multiple Spawning Simulations. Phys. Chem. Chem. Phys. 2014, 16, 11770-11779. [30] Craig, N. C.; Groner, P.; McKean, D. C. Equilibrium Structures for Butadiene and Ethylene: Compelling Evidence for Electron Delocalization in Butadiene. J. Phys. Chem. A 2006, 110, 7461-7469. [31] Klein, L. B.; Morsing, T. J.; Livingstone, R. A.; Townsend, D.; Solling, T. I., The Effects of Symmetry and Rigidity on Non-Adiabatic Dynamics in Tertiary Amines: A TimeResolved Photoelectron Velocity Map Imaging Study of the Cage-Amine ABCO. Phys. Chem. Chem. Phys. 2016, 18, 9715-9723. [32] Parker, D. H.; Avouris, P. Multiphoton Ionization Spectra of Two Caged Amines. Chem. Phys. Lett. 1978, 52, 515-520. [33] Disselkamp, R.; Shang Q.-Y.; Bernstein, E. R. CASSCF Study of the Ground State and Lowest Lying 3s Rydberg States of ABCO. J. Phys. Chem. 1995, 99, 7227-7230. [34] O. Schalk, O.; Schuurman, M. S.; Wu, G.; Lang, P.; Mucke, M.; Feifel, R.; Stolow, A. Internal Conversion versus Intersystem Crossing: What Drives the Gas Phase Dynamics of cyclic α,β-enones? J. Phys. Chem. A 2014, 118, 2279-2287.



[35] Schalk, O.; Unterreiner, A.-N. Transient Anisotropy in Degenerate Systems: A SemiClassical Approach. Z. Phys. Chem. 2011, 225, 927-938. [36] Vormann, K.; Dreizler, H. Quadrupole Hyperfine Structure in the Rotational Spectrum of Quinuclidine. J. Mol. Struct. 1988, 190, 489–492. [37] Boguslavskiy, A. E.; Schuurman, M. S.; Townsend, D.; Stolow, A. Non-BornOppenheimer Wavepacket Dynamics in Polyatomic Molecules: Vibrations at Conical Intersections in DABCO. Faraday Discuss. 2011, 150, 419-438.


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Figure 1. One color signal of butadiene ionized with two photons of 267 nm integrated between 0.15 and 0.35 eV photoelectron kinetic energy in dependence of the polarization angle. 215x165mm (144 x 144 DPI)



Figure 2. Time resolved photoelectron signal of pyrazine pumped at 324 nm and probed at 400 nm. The time scale is linear during the first picosecond and exponential at longer time delays. 264x186mm (144 x 144 DPI)


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Figure 3. a) Time resolved photoelectron signal of pyrazine pumped at 324 nm and probed at 401 nm with pump and probe pulses polarized parallel (black squares) and perpendicular (red circles) with respect to each other, measured at a photoelectron kinetic energy of 0.58 eV. b) Anisotropies calculated from the signal in panel a (black squares) and at photoelectron energies of 0.02 eV (blue circles) and 3.0 eV (dark yellow triangles). 254x188mm (144 x 144 DPI)



Figure 4. a) Time-resolved photoelectron spectrum 1,3-butadiene upon excitation with two photons at 400 nm and probe at 267 nm measured for parallel pump and probe pulses. b) Photoelectron anisotropy. 105x80mm (144 x 144 DPI)


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Figure 5. a) Time-resolved photoelectron spectrum 1,3-butadiene upon one photon excitation at 216 nm and probe at 267 nm measured for parallel pump and probe pulses. b) Photoelectron anisotropy. The region between 1.7 and 2.3 eV is not displayed because the background of the data taken with perpendicularly polarized beams was too high. 157x127mm (144 x 144 DPI)



Figure 6. Anisotropy traces of 1-azabicyclo[2.2.0]octane (ABCO) after excitation at either 228.6 nm (panels a and c) or 223.6 nm (panels b and d). The black squares symbolize the anisotropies calculated from the whole S2 and S1 band while the thin lines represent selected energies integrated over regions ± 5 meV around the given value. 143x99mm (144 x 144 DPI)


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TOC Graphic 95x82mm (144 x 144 DPI)


Anisotropy in Time-Resolved Photoelectron Spectroscopy in the Gas

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