(a) Transient spectrum of HDO in (dots) directly after excitation and after vibrational relaxation . (b) Transient spectrum of HDO in (dots) directly after excitation and after vibrational relaxation . The solid lines are fits to the relaxation model that is described in the text.
Delay curves at three frequencies in the transient spectrum of the OH vibration of HDO. The solid lines are fits to the relaxation model discussed in the text. (a) At the signal consists of a decaying bleach. (b) At a competition is observed between a decaying bleach and an ingrowing heating signal. (c) At the signal consists of a decaying induced absorption.
Relaxation mechanism of the OH-stretch vibration of HDO in and of the OD-stretch vibration of HDO in . The first step in the relaxation is the decay of the hydroxyl stretching vibration to an intermediate level that has the same spectrum as the ground state. The next step is the relaxation of the intermediate level to the heated ground state .
(a) Anisotropy decay of HDO in at constructed from raw data that were corrected for heating in two different ways. In the first method (solid circles) the constant end level is subtracted from the raw data. The second method (open squares) subtracts the time-dependent heating signal. The solid lines represent monoexponential fits to the data points. The first method leads to a reorientation time of , and the second method to a reorientation time of . (b) Anisotropy decay of HDO in at . The raw data have been corrected for the ingrowing heating signal, but nevertheless a flawed anisotropy is obtained.
[(a) and (b)] Parallel (open squares) and perpendicular signals (solid circles) for HDO in (after correction for heating) at 3410 and . The dashed line represents the isotropic signal. The solid lines are fits to the model that is described in the text. [(c) and (d)] Anisotropy decays associated with the signals in (a) and (b).
[(a) and (b)] The same plots as those in Fig. 5(a) and 5(b) but with the axis expanded. The open squares represent the parallel signal, the solid circles represent the perpendicular signal, the dashed line is the isotropic signal, and the solid lines are fits to the model that is described in the text.
Difference between the parallel and perpendicular signals of the OH vibration at a delay of . The top panel shows the transient spectrum at a delay of . Apparently, the parallel signal is lower than the perpendicular signal over the entire spectrum, even in the region of the transient spectrum that has the character of an induced absorption.
Illustration of the effect that leads to a bleach in the parallel signal and an induced absorption in the perpendicular signal, at a time delay for which vibrational relaxation is complete. (a) The pump beam creates a distribution of excited molecules. (b) At the same time an anisotropic “hole” is created in the distribution of ground state molecules. (c) After a few picoseconds the orientational distribution of initially excited molecules becomes more isotropic. (d) The hole in the distribution of ground state molecules also becomes more isotropic; however, we assume that ground state molecules reorient more slowly than excited molecules. (e) Adding the two contributions results in a distribution with more molecules perpendicular to the pump than parallel to it.
(a) Relaxation scheme of a system that relaxes from level to . (b) Integration scheme used to calculate the anisotropy of level . Until the molecule is in level and its correlation function decreases by a factor of . From to the molecule resides in level , and as a consequence its correlation function further decreases by a factor of . In order to find the anisotropy of level one must integrate over all possible times at which the molecule decays from level to .
Anisotropy decay of HDO in at the maximum of the bleach . The correction procedure described in the text has been employed (dots). For comparison the uncorrected anisotropy is also shown (open squares). The solid line is a monoexponential fit.
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