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Femtosecond pump-probe photoionization-photofragmentation spectroscopy: Photoionization-induced twisting and coherent vibrational motion of azobenzene cation
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Image of FIG. 1.
FIG. 1.

Schematic diagram showing the concept of femtosecond pump-probe photoionization-PF spectroscopy. ABC: neutral ground state; : cation lower state; : cation upper excited state; : fragment ion. The solid curved arrow represents relaxation process of the lower state and the dotted arrows represent possible fragmentation routes. FC active coordinates represent those exhibiting significant geometric difference between the neutral ground state and the cation lower state.

Image of FIG. 2.
FIG. 2.

Typical transients obtained by monitoring AZB parent-ion signal under the low-pump-irradiance condition for (a) ; (b) , 305, and 266 nm. Probe wavelengths were 400 nm for all cases. The inset in (a) shows a vapor-phase absorption spectrum of taken from Ref. 56. The inset in (b) shows a biexponential decay fit to the 330 nm transient. A depletion component was included in the fit to account for the very small residual photoionization-PF contribution (see text). The decay times of the two components were fixed at the values reported in Ref. 37, and therefore, the fit is not the best fit and only meant to show that our photoion transients are consistent with the photoelectron measurements.

Image of FIG. 3.
FIG. 3.

AZB transient recorded with three pump-laser pulse energies: (a) , (b) 1.7, and (c) . The probe pulse energy was kept constant . The mutual polarization of the pump (266 nm) and probe (400 nm) beams was set at the magic angle (54.7°). The data are neither normalized nor shifted in order to faithfully show the effect of the pump irradiance. The zero signal level (dashed line), the background signal level due to probe laser alone (dashed-dot line), and the background signal level due to pump laser alone for trace (c) (dashed-dot-dot line) are also indicated in the figure.

Image of FIG. 4.
FIG. 4.

AZB transients measured at several pump wavelengths under the high-pump-irradiance condition. From top to bottom, the pump wavelengths are 260, 330, 340, 350, 360, and 450 nm. The probe wavelength was 400 nm, except in the case of 260 nm pumping where 390 nm was used as the probe. The pump pulse energies are for 450 nm, for 360–330 nm, and for 260 nm. These transients are normalized and shifted vertically for the sake of clarity. Notice that the oscillation periods are almost identical for all pump wavelengths.

Image of FIG. 5.
FIG. 5.

Background-subtracted TOF-MS spectra measured at several pump-probe delay times from rear to front: 0, 0.5, 1.0, 1.5, 2.0, and 2.5 ps. Each trace was obtained by subtracting the background mass spectrum measured at a negative delay time from that measured at the indicated positive delay time. The mutual polarization of the pump and probe beams was set at the magic angle (54.7°). Negative-going peaks indicate ion depletion and positive-going peaks indicate ion formation. The dashed curves are just to guide the eye.

Image of FIG. 6.
FIG. 6.

Transients measured under identical conditions for (a) AZB parent ion and (b) phenyl fragment ion. In both cases, , ; , . The absence of the positive-going sharp feature near zero delay time in the AZB transient is due to the very low probe pulse energy used in this case.

Image of FIG. 7.
FIG. 7.

Schematic diagrams showing the two pump-probe schemes discussed in the text: (a) the excitation-MPI pump-probe scheme and (b) the photoionization-PF pump-probe scheme.

Image of FIG. 8.
FIG. 8.

Transients recorded with (a) and (b) . The pump wavelengths are the same (350 nm) for both traces. Enlargements of the oscillatory behaviors are also shown with the multiplication factors indicated. Note the phase shift in oscillation between the two transients.

Image of FIG. 9.
FIG. 9.

Polarization dependence of the AZB transients taken with (a) high probe pulse energy and (b) low probe pulse energy . In both cases, and . The mutual polarization of the pump and probe beams for each trace is as indicated. Time-dependent anisotropy functions derived from these data are shown in the insets. Rapid oscillation in derived from traces (a) and (c) due to the interference of the initial sharp features is not shown.

Image of FIG. 10.
FIG. 10.

Best fit (solid line) to a typical AZB transient measured under high-pump-irradiance condition with and . The dashed lines are the decomposed components included in the fitting model as described in the text. The mutual polarization of the pump and probe beams was set at the magic angle (54.7°) in order to minimize the rotational effect.

Image of FIG. 11.
FIG. 11.

Optimized molecular structures of AZB neutral ground state (left panel) and cation ground state (right panel) at the level of theory.

Image of FIG. 12.
FIG. 12.

(a) Calculated potential energy curves (see text) for the neutral and cation ground-state minimum-energy path along the CNNC torsional coordinate. The wave packet and the relaxation path (curved arrows) are schematic. Notice that the energy scales for the lower and upper regions of the figure are very different. (b) Displacement vectors of the imaginary-frequency mode corresponding to reaction coordinate at the cation twisting transition state.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Femtosecond pump-probe photoionization-photofragmentation spectroscopy: Photoionization-induced twisting and coherent vibrational motion of azobenzene cation