Charles S. Parmenter

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...

On the contributions of van der Waals interactions to vibrational level mixing. Torsion–vibration coupling in p‐fluorotoluene

As an example demonstrating the importance of van der Waals interactions to vibrational level mixing, a model is developed for the intramolecular coupling of internal methyl torsion to aromatic ring vibrations in p‐fluorotoluene (pFT). The coupling is a consequence of van der Waals interactions between the methyl rotor and the ring. Fermi resonance matrix elements are generated in a manner analogous to previous calculations for collision‐induced vibrational energy transfer and van der Waals molecule predissociation. Using these matrix elements, the extent of vibrational mixing is calculated for the level 6a1 in S1 pFT. The calculated result is in agreement with spectroscopic observations. Arguments for the generality of van der Waals interactions as mediators of intramolecular vibrational level mixing are presented.

p‐fluorotoluene. I. Methyl (CH3 and CD3) internal rotation in the S1 and S0 states

Supersonic jet S1‐S0 spectroscopy (resonance‐enhanced multiphoton ionization, fluorescence excitation, and dispersed single vibronic level fluorescence) has been used to determine the S1 and S0 internal rotation energy level structure of p‐fluorotoluene with a CD3 methyl rotor as well as to extend observations of the CH3 rotor structure. The observed rotor energy levels 2≤m≤8 for both species in both states are fit by a simple sixfold hindered rotor Hamiltonian for which the rotor inertial constants B and the internal rotation potential energy barriers V6 are evaluated. V6 may be obtained independently from B by observations of ΔE3, the observed splitting of the 3a‘1 and 3a‘2 rotor levels. Numerical solution of the wave equation shows that the perturbation theory relationship V6=−2ΔE3 holds well for any reasonable B value. Correspondingly, the B constant may be obtained from other level energies without appreciable sensitivity to (reasonably) assumed barrier heights. Earlier microwave and S1‐S0 fluorescen...

Mode-to-mode vibrational energy flow in S1 benzene☆

Abstract Resolved fluorescence spectra from low pressures of benzene with nine added gases have been used to follow mode-to-mode vibrational relaxation in the S 1 state of benzene under “single-collision” conditions. Cw pumping of the S 1 fundamental 6 1 (ν″ 6 = 522 cm −1 ) allows study of collisional vibrational energy flow into each of four channels. Two channels consist of flow into single levels, and the others represent flow into unresolved pairs of levels. The mode-to-mode cross sections are much larger than those usually observed in ground electronic states, being near gas kinetic even for partners transferring energy by VT, R processes alone. The mode-to-mode transfer has highly specific patterns, with roughly seventy percent of the transfer going into the four channels in spite of many other nearby levels. The largest cross sections are always to a level 237 cm −1 above the initial level rather than to a level nearly resonant (Δ E = 7 cm −1 ) with the initia l level. A common pattern of flow occurs for the four gases transferring energy by VT, R processes alone, and another common pattern is established for the five gases which can also use VV transfers. With the exception of one channel, VV resonances with vibrationally complex partners increase cross sections by less than a factor of two over that provided by the VT, R path. VV transfers have a similarly small effect on the overall vibrational relaxation rate out of the initial level 6 1 . Both the flow patterns and the VV versus VT, R competitions are accounted for with an extremely simple and general set of propensity rules taken directly from SSH calculations made by others for vibrational relaxation in ground electronic states. The rules are based on the degeneracies of the final levels, the number of vibrational quantum changes, and the amount of energy exchanged between vibrational and translational/rotational degrees of freedom. The rules seem general to relaxation in both ground and excited electronic states, whereas large cross sections seem a special property of the excited state. The cross sections for collision partners SF 6 and perfluorohexane are small relative to those for other partners with similar vibrational complexity and mass.

n,π* fluorescence from selected vibronic levels of pyrimidine vapor: Franck–Condon factors and excited state anharmonic coupling

An analysis is presented of fluorescence from various selected vibrational levels in the 1B1(S1) state of pyrimidine vapor. The intensity distribution in fluorescence from the zero point level establishes with security the general pattern of vibrational activity in the 1B1–1A1 transition, and the Franck–Condon intensities in this fluorescence set the molecular parameters required for calculation of Franck–Condon intensities in fluorescence from other levels. The fluorescence from every higher level so far reached shows marked deviations from the predicted Franck–Condon intensities. These deviations can be attributed to strong (ca., 30 cm−1) anharmonic coupling in the excited state. Every observed totally symmetric fundamental in the 1B1 state is perturbed by this mixing. Calculations which include the coupling provide a successful account of the perturbed Franck–Condon intensities. The fluorescence analyses secure the assignment of a number of absorption bands as well as several excited state fundamentals...

Radiative and nonradiative processes in the first excited singlet state of azabenzene vapors

Measurements of fluorescence quenching, fluorescece quantum yields, and triplet formation quantum yields are combined with data in the literature to construct a general picture of S 1 decay in the six azabenzene vapours pyridine, pyrazine, pyrimidine, pyridazine, s -triazine and s -tetrazine. Attention has been directed especially to S 1 -T crossing from the S 1 zero point level, except for s -triazine in which the crossing from the 677 cm −1 fundamental ( v ′ 6 ) has been characterized. No evidence for triplet formation is found for pyridazine and s -tetrazine. A fast collision-free channel leading ultimately to dissociation probably dominates S 1 decay in these molecules. Triplet formation occurs in the remaining four azabenzenes, and the fluorescence quenching in three (pyridine fluorescene could not be detected) shows that in those cases the S 1 -T crossing is collision induced. Firm evidence concerning the effect of collisions on triplet formation in pyridine could not be obtained. The triplet yields reach a maximum limiting value as added gas pressures become high enough so that collisional channels dominate S 1 decay. This value is unity in pyrazine and pyrimidine; in pyridine and s -triazine it is about 0.5. An unidentified collisional channel competes effectively with collision-induced crossing in s -triazine and probably in pyridine as well. The fluorescence quantum yields extrapolated to zero pressure are less than 10 −2 in every azabenzene except pyrimidine where the yield is ⩾0.3. Pyrazine remains the only azabenzene vapor in which phosphorescence has been observed.

Acceleration of intramolecular vibrational redistribution by methyl internal rotation. A chemical timing study of p-fluorotoluene and p-fluorotoluene-d3

Time‐integrated, frequency‐resolved fluorescence spectroscopy has been used to determine rates of intramolecular vibrational energy redistribution (IVR) from the vibrational levels 31 (evib≊1200 cm−1) and 3151 (evib≊2000 cm−1) in both p‐fluorotoluene and p‐fluorotoluene‐d3 for comparison with each other and with comparable levels in p‐difluorobenzene. Methyl substitution increases the rate of IVR by roughly two orders of magnitude, while deuteration of the methyl rotor produces at most a small (two‐ to fourfold) further increase in the rate of IVR. It is argued that the IVR response to methyl substitution is a consequence of the methyl internal rotation without significant influence from the methyl vibrations. The increased IVR rate is predominantly a reflection of the large number of additional states that can couple through the exchange of energy between ring vibration and internal rotation. The difference, if any, between the protonated and deuterated methyl rotor species probably arises from subtle differences in the level structures and coupling strengths of the two systems. Fermi golden rule modeling of the relative IVR rates is built on these propositions. It accounts for much of the IVR rate increase associated with the methyl substitution as well as for the near equivalence of the –CH3 and –CD3 IVR rates.Time‐integrated, frequency‐resolved fluorescence spectroscopy has been used to determine rates of intramolecular vibrational energy redistribution (IVR) from the vibrational levels 31 (evib≊1200 cm−1) and 3151 (evib≊2000 cm−1) in both p‐fluorotoluene and p‐fluorotoluene‐d3 for comparison with each other and with comparable levels in p‐difluorobenzene. Methyl substitution increases the rate of IVR by roughly two orders of magnitude, while deuteration of the methyl rotor produces at most a small (two‐ to fourfold) further increase in the rate of IVR. It is argued that the IVR response to methyl substitution is a consequence of the methyl internal rotation without significant influence from the methyl vibrations. The increased IVR rate is predominantly a reflection of the large number of additional states that can couple through the exchange of energy between ring vibration and internal rotation. The difference, if any, between the protonated and deuterated methyl rotor species probably arises from subtle di...

Single vibronic level fluorescence. III. Fluorescence yields from three vibronic levels in the 1B2u state of benzene

Abstract Absoute quantum yields of luorescence have been determined for each of three specific vibronic levels in isolated 1B2u benzene molecules. The yields are 0.22, 0.27 and 0.25 for the zero point level and for the state ν16 and (ν16 + ν11) respectively. The methods used to derive these yields are described.

Fluorescence, Phosphorescence, and Triplet Formation in Biacetyl at Low Pressures

The S1 → T intersystem crossing in biacetyl vapor has been examined by direct observation of emission from both the initial and final states. When biacetyl is excited by absorption of 4358‐A radiation, both the singlet‐ and the triplet‐emission yields are independent of pressure in the range 40–0.1 torr. Since isolated molecule conditions are established for singlet relaxation in the lower limit of this range, triplet formation must occur in biacetyl in the absence of collisional perturbations involving the excited singlet state. Triplet emission continues to be observable at pressures descending to 0.03 torr, but the emission yield decreases due to triplet deactivation at the walls. The results are discussed within the framework of recent theories of radiationless transitions.

Collision induced intersystem crossing the photophysics of glyoxal vapor excited at 4358 Å

Abstract The yields, lifetimes and spectra of singlet 1Au (S1) and triplet 3Au (T1) emissions from glyoxal vapor (0.003 to 10 torr) have been measured after initially pumping levels about 1000 cm−1 above the S1 zero-point level with the 4358 A Hg line and with flash excitation centered at 4345 A. Only S1 emission is observed at the lowest pressures. The singlet fluorescence contains appreciable structure from the zero-point level even when the hard sphere collision interval exceeds the radiative lifetime calculated from the absorption coefficient. Implications of long lifetimes (due to S1 - T1 vibronic interactions) are not confirmed by pulsed excitation studies. Both S1 and T1 emissions are observed at pressures above about 0.1 tert and both are self-quenched. However, added gases such as cyclohexane, argon, and helium selectively quench only S1 emission. This quenching is collision-induced S1→T1 intersystem crossing with cross sections of order 0.1 hard sphere for transitions from the S1 zero-point level. The triplet yield in 0.2 torr of pure glyoxal is probably near unity, and the subsequent crossing T1 → S0, if it occurs, lies in the statistical limit. Indications of fast nonradiative decay from high triplet vibrational levels are seen in the phosphorescence yields. Self-quenching of the triplet state appears to be associated with the photochemical activity of glyoxal.