Abstract:The uniform theory of photoemission of a dissociating molecule excited by a laser pulse is developed. It is shown that transient effects lead to the appearance of additional terms, not included in the Kramers-Heisenberg formula. The role of "true" Raman scattering vs "resonance fluorescence" in contributing to the observed signal is elucidated. It is shown that the relative importance of these two processes is strongly dependent on the pulse parameters and the radiative lifetimes. "True" Raman is shown to domi… Show more
“…Polarization analysis at 266 and 280 nm found depolarization ratios for all 3 n to be close to 1 / 3 , the value expected for a parallel transition to one or more excited states. , It was found that the observed REP's could all be fit reasonably well by a model in which the 3 1 fundamental was subject to off-resonant interference from a higher electronic state or states with parallel transition dipole character. (An alternative explanation was given by Shapiro, although off-resonant interference had also been identified in other alkyl iodide studies, see next section.) Since off-resonant scattering primarily interferes with fundamentals due to the shortness of its duration, the overtone and combination band REP's could be used for extraction of the details of the resonant PES in the model.…”
Resonance Raman spectroscopy as a probe of the early stages in the dissociation dynamics of polyatomic molecules has become a valuable complement to photofragmentation studies. While these spontaneous Raman experiments are obtained in the frequency domain, they often reflect evolution of the molecule during the first few femtoseconds of bond breaking. Coupled with progress in classical and quantum calculations of large-amplitude motion, unique insights have become available for a number of small polyatomic molecules. The development of this field to date is reviewed.
“…Polarization analysis at 266 and 280 nm found depolarization ratios for all 3 n to be close to 1 / 3 , the value expected for a parallel transition to one or more excited states. , It was found that the observed REP's could all be fit reasonably well by a model in which the 3 1 fundamental was subject to off-resonant interference from a higher electronic state or states with parallel transition dipole character. (An alternative explanation was given by Shapiro, although off-resonant interference had also been identified in other alkyl iodide studies, see next section.) Since off-resonant scattering primarily interferes with fundamentals due to the shortness of its duration, the overtone and combination band REP's could be used for extraction of the details of the resonant PES in the model.…”
Resonance Raman spectroscopy as a probe of the early stages in the dissociation dynamics of polyatomic molecules has become a valuable complement to photofragmentation studies. While these spontaneous Raman experiments are obtained in the frequency domain, they often reflect evolution of the molecule during the first few femtoseconds of bond breaking. Coupled with progress in classical and quantum calculations of large-amplitude motion, unique insights have become available for a number of small polyatomic molecules. The development of this field to date is reviewed.
“…The interference from higher electronic states was approximately taken into account by adding contributions to the polarizabilities of the sort with adjustable parameters D or , E or , and Γ or . Other attempts to model the off-resonant electronic interference have been discussed by Galica et al and by Phillips and Myers, and Shapiro has provided a alternative rationalization of the 3 1 REP structure. 3 Raman excitation profiles for 3 n fundamental and overtones measured by Galica et al , and results of current best fit. 4 Raman excitation profiles for 2 1 3 n fundamental and combination bands measured by Galica et al and results of current best fit.…”
The broad structureless A band of CH 3 I has long been assigned to three overlapping transitions, although their relative strengths have been a subject of controversy. A new component analysis is made utilizing a simple dynamical model to rationalize the currently available absorption, magnetic circular dichroism, and resonance Raman excitation spectral data. Discrepancies between these and other CH 3 I photolysis experiments are examined.
“…Methyl iodide has been extensively studied as a test case for photoreaction dynamics of polyatomic molecules. A voluminous body of information, in both theory − and experiment, ,,− is thus available to us. This lode of information includes calculated potential energy surfaces 25-27 and experimental characterization of the photochemistry of methyl iodide clusters, ,,− both of which we draw from in the investigation that follows.…”
Due to the implication of iodine chemistry in stratospheric ozone
depletion, an accurate atmospheric lifetime
of methyl iodide has recently become of interest. To calculate
this lifetime, a reliable temperature-dependent
UV photodissociation cross section, in the region that overlaps with
available solar light, is vital. Unfortunately,
measurement of this cross section is complicated by the fact that, at
typical laboratory pressures, methyl
iodide readily forms dimers whose ultraviolet absorption differs from
that of the monomer and that dimer
formation is also temperature-dependent. We use a combination of
theory and experiment to separate the
changes in the absorption due to the temperature dependence of dimer
formation from the narrowing of the
absorption band that results from rotational and vibrational cooling of
isolated methyl iodide molecules.
Calculation of the predicted absorption cross section shows that
the valence band absorption spectrum narrows
only slightly upon cooling from 25 to −73 °C (200 K).
Absorption spectra were also measured experimentally
at a range of pressures from 0.1 to 2.4 Torr and a range of
temperatures from −22 to 100 °C. The temperature-dependent cross section measured at 0.1 Torr agrees well with the
calculated temperature dependence. The
spectra at higher pressures show strong pressure as well as temperature
dependence. This pressure dependence
allowed us to constrain the temperature-dependent equilibrium constant
for dimer formation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
