Alan E. W. Knight

The role of intermolecular potential well depths in collision‐induced state changes

A relationship is developed from two distinct theoretical approaches to correlate the rate constants kM or cross sections σM for a series of added gases M which collisionally induce a state transformation A*→B. The correlation derived from theory is where C is a constant and eA*M is the intermolecular well depth between A* and M. We observe that experimental data can be described by a related correlation where β is a constant and eMM is the well depth between pairs of M molecules. This correlation is shown to be general. It works for electronic state deactivation in atoms, intersystem crossing and internal conversion in S1 polyatomics, rotational and also vibrational relaxation in S1 polyatomics, predissociation in diatomics and polyatomics, and vibrational relaxation in a free radical as well as in a molecular ion. The theory is appropriate only when attractive forces dominate the interaction, and this seems consistent with the experimental data. The correlation thus provides a simple means to distinguish between attractive and repulsive interactions. The correlation also reveals that collision partners do not substantially modify the intrinsic S1‐T mixing during collision‐induced intersystem crossing.A relationship is developed from two distinct theoretical approaches to correlate the rate constants kM or cross sections σM for a series of added gases M which collisionally induce a state transformation A*→B. The correlation derived from theory is where C is a constant and eA*M is the intermolecular well depth between A* and M. We observe that experimental data can be described by a related correlation where β is a constant and eMM is the well depth between pairs of M molecules. This correlation is shown to be general. It works for electronic state deactivation in atoms, intersystem crossing and internal conversion in S1 polyatomics, rotational and also vibrational relaxation in S1 polyatomics, predissociation in diatomics and polyatomics, and vibrational relaxation in a free radical as well as in a molecular ion. The theory is appropriate only when attractive forces dominate the interaction, and this seems consistent with the experimental data. The correlation thus provides a simple means to distinguis...

Stretch–bend coupling between van der Waals modes in the S1 state of substituted benzene–Ar1 complexes

The van der Waals vibrations of aniline–, phenol–, fluorobenzene–, and chlorobenzene–Ar1 complexes have been measured using one‐color resonance enhanced multiphoton ionization spectroscopy, together with time‐of‐flight mass spectrometry, in a skimmed supersonic molecular beam. A delayed ionization extraction technique is used to suppress contributions to the spectra from dissociating complexes. The S1–S0 electronic origins for the van der Waals complexes are found to be shifted towards lower energy (red shift) relative to the parent molecule electronic origin for all the Ar1 complexes. The red shifts increase in magnitude in the order: fluorobenzene, chlorobenzene, phenol, aniline. Progressions, overtones and combination transitions involving the low frequency van der Waals vibrations, i.e., the symmetric bend (bx), the asymmetric bend (by) and the stretch (sz) are observed clearly in the S1←S0 excitation spectra. Intensity profiles are found to deviate substantially from those expected on the basis of ha...

The S1–S0(1B2u–1Ag) transition of p‐difluorobenzene cooled in a supersonic free jet expansion. Excitation and dispersed fluorescence spectra, vibrational assignments, Fermi resonances, and forbidden transitions

The vibronic spectroscopy of the S1(1B2u)–S0(1Ag) transition of p‐difluorobenzene (000 at 36 838 cm−1) cooled in a supersonic free jet expansion in argon has been reinvestigated in some detail. Analysis of over 50 vibronic transitions using fluorescence excitation and dispersed single vibronic level fluorescence spectroscopy has led to the establishment or confirmation of the assignments of 19 S1 and S0 frequencies, including eight previously unassigned S1 vibrational frequencies, and the reassignment of two S1 and one S0 frequencies. Several Franck–Condon forbidden transitions have been identified. Their activity in the S1–S0 spectrum is attributed to vibronic coupling involving higher lying electronic states. Forbidden transitions involving b3g modes, notably ν27 and ν26, derive their intensity from a higher lying 1B1u electronic state, via vibronic coupling that is analogous to that responsible for the 1B2u–1Ag transition in benzene. Numerous Fermi resonances in both the S1 and S0 states have been iden...

Wide fluctuations in fluorescence lifetimes of individual rovibronic levels in SiH2 (Ã 1B1)

We have measured fluorescence lifetimes of individual rovibronic levels in SiH2 (A 1B1, 020). The lifetimes vary widely from one level to the next, ranging from ≲10 ns to >1 μs. Similar behavior is seen in the (000), (010), and (030) levels. This behavior is interpreted in terms of coupling of the A 1B1 rovibronic levels with background levels in the X 1A1 (S0) and a 3B1 (T1) states, and thence via predissociation to Si(3P)+H2. The irregular variation in the lifetimes reflects the quasirandom spacings of S0 and T1 levels in the vicinity of any particular A state level.

The van der Waals vibrations of aniline-(argon)2 in the S1 electronic state

Vibrational structure associated with van der Waals modes of the aniline–(argon)2 complex has been observed in the region near the origin of the S1←S0 electronic transition of the complex using resonance enhanced, multiphoton ionization (REMPI) spectroscopy. The aniline–Ar2 spectrum in this region displays only a few discrete bands built on an intense electronic origin. The dominant vibrational band, associated principally with the symmetric van der Waals stretching motion of the two argon atoms against the aromatic frame, occurs at 38.5 cm−1 displacement relative to the 000 band, with weaker transitions at 15 and 30 cm−1 displacement. A simple model for the van der Waals vibrations of aromatics bound to one and two rare gas atoms is developed and allows allow us to explain the aniline–Ar2 spectrum, using van der Waals bond parameters determined from the previously measured An–Ar1 spectrum. The agreement between the predicted and observed aniline–Ar2 spectrum confirms the view that van der Waals stretchin...

Overtone spectroscopy of H2O clusters in the vOH=2 manifold: Infrared-ultraviolet vibrationally mediated dissociation studies

Spectroscopy and predissociation dynamics of (H2O)2 and Ar-H2O are investigated with vibrationally mediated dissociation (VMD) techniques, wherein upsilon(OH) = 2 overtones of the complexes are selectively prepared with direct infrared pumping, followed by 193 nm photolysis of the excited H2O molecules. As a function of relative laser timing, the photolysis breaks H2O into OH and H fragments either (i) directly inside the complex or (ii) after the complex undergoes vibrational predissociation, with the nascent quantum state distribution of the OH photofragment probed via laser-induced fluorescence. This capability provides the first rotationally resolved spectroscopic analysis of (H2O)2 in the first overtone region and vibrational predissociation dynamics of water dimer and Ar-water clusters. The sensitivity of the VMD approach permits several upsilon(OH) = 2 overtone bands to be observed, the spectroscopic assignment of which is discussed in the context of recent anharmonic theoretical calculations.

Vibrational relaxation in ground state p‐difluorobenzene: Single level preparation by stimulated emission pumping

We have measured the cross section for vibrational relaxation from the 52302 level (evib = 2036 cm−1) of S0 p‐difluorobenzene (pDFB) due to pDFB‐pDFB collisions. Stimulated emission pumping has been used to selectively populate the 52302 level. Single vibronic level fluorescence from the S1 level 302, generated by exciting the 5023022 transition at a variable delay time after 52302 has been populated, has been used to monitor the population dynamics of 52302. We obtain a value of 4±1×107s−1 Torr−1 for the vibrational relaxation rate coefficient, a value which is 2–3 times the hard sphere value. This large cross section is consistent with the cross sections observed in S1 benzene but is an order of magnitude larger than the cross sections observed previously for a variety of molecules in the ground electronic state. (AIP)

Mass selected resonance enhanced multiphoton ionization spectroscopy of aniline-Arn(n=3,4,5,...) van der Waals complexes

The origin region of the S1←S0 transitions of the aniline–Ar3, aniline–Ar44, and aniline–Ar5 molecules have been measured using mass selected resonance enhanced, multiphoton ionization (REMPI) spectroscopy. The aniline–Ar3 spectrum exhibits two distinct groups of peaks. The more prominent group displays a regular vibrational progression, with five obvious members and a spacing of ∼16 cm−1. Vibrational structure in the other group is less distinctive. On the basis of cluster potential calculations described in this paper, we believe that two stable aniline–(argon)3 isomers exist in the supersonic expansion and that the two groups of peaks correspond to absorption by these two isomers. Spectra recorded at masses corresponding to aniline–(argon)4 and aniline–(argon)5 display broadened structure that probably reflects contributions from larger aniline–(argon)n clusters which fragment upon ionization. There is, however, some evidence for a progression with a spacing of ∼16 cm−1 in the aniline–(argon)4 spectrum...

Collisional deactivation of a selected energy level in S0 p‐difluorobenzene embedded in a dense vibrational field: Absolute rate constants for a variety of collision partners

Absolute rate constants are reported for collisional relaxation from the 52302 level (evib=2036 cm−1) of S0(1Ag) p‐difluorobenzene into the relatively dense surrounding field of S0 vibrational levels (6–8 per cm−1). The relaxation is induced by a bath of foreign gas at 300 K under bulb conditions. A total of 23 collision partners have been studied. Stimulated emission pumping (SEP) is used to populate the 52302 level selectively. Single vibronic level fluorescence (SVLF) from the S1 level 302, generated by exciting the 5023022 transition at a variable delay time (85–260 ns) after 52302 preparation, is used to monitor the population dynamics of 52302. The equivalent cross sections deduced from the rate constants for vibrational relaxation with foreign gas M range from one quarter of the hard sphere value for M=He to 2.7 times hard sphere for M=(C2H5)2O. These are an order of magnitude larger than the cross sections normally found for vibrational relaxation in ground electronic states of smaller molecules, ...

Suppression of fragment contributions to mass‐selected resonance enhanced multiphoton ionization spectra of van der Waals clusters

A technique is described for suppressing the contribution due to fragmentation of higher mass clusters in mass‐selected resonance enhanced multiphoton ionization spectroscopy of weakly bound van der Waals (vdW) clusters formed in a skimmed supersonic expansion. Suppression is achieved by ionizing upstream from the extraction region of a time‐of‐flight mass spectrometer and by using a delayed voltage pulse to extract the ions. Normally, spectra associated with a particular cluster ion, e.g., AB+n will contain contributions from fragments formed by the dissociation of higher mass clusters, e.g., AB+n+m. However, with upstream ionization and delay extraction, AB+n fragments formed by AB+n+m dissociation that have different kinetic energy and altered trajectories from the directly ionized AB+n clusters, are not extracted efficiently and are hence discriminated against in detection. The technique is demonstrated with the aniline–argon vdW complex.