W. M. McClain

Polarization and assignment of the two-photon excitation spectrum of benzene vapor☆

Abstract The two-photon excitation (TPE) of benzene fluorescence in the vapor phase at 60 torr is reported for the total-energy region from 38 086 cm −1 to 42 441 cm −1 using both circular and linear polarized light from a nitrogen-pumped dye-laser. The theory of the polarization dependence of the vibronic transitions in benzene is briefly reviewed, and it is seen how transitions involving vibrations of b 1u symmetry are expressly forbidden for this type of TPE experiment in which the two photons are identical. Five vibronic origins with distinctive rotational contours and polarization dependence are identified in the TPE spectrum. The υ 14 (b 2u ) vibronic origin at 1570 cm −1 (above the electronic origin of the I B 2u state) stands out very prominently in the linear polarized spectrum, but nearly disappears in the circular polarized spectrum. This striking polarization dependence indicates a significant contribution of A 2u electronic states to the intermediate states of this TPE vibronic transition. The relatively great strength of the υ 14 band may be due to vibronic borrowing by the b 2u mode from the ground electronic state (A 1g ).

Two-photon excitation spectra of the low energy excited states of diphenylhexatriene and diphenyloctatetraene☆

Abstract All- trans diphenylhexatriene and diphenyloctatetraene in solution show a very strong two-photon absorption beginning at about the same energy as the one-photon spectrum. This supports the postulated presence of a low-lying 1 A g electronic state.

The effect of spatial and temporal laser beam characteristics on two-photon absorption

Abstract We present calculations which indicate possible sources of error in the determination of two-photon absorptivity introduced by the choice of experimental conditions. Both the direct absorption experiment and the two-photon excitation experiment are considered. We include the effects of beam alignment, diffraction-limited focusing, and pulse width variation on the observed two-photon absorptivity.

Detection and assignment of the ’’phantom’’ photochemical singlet of trans‐stilbene by two‐photon excitation

We have made a complete polarization study of the two‐identical‐photon excitation (TIPE) spectrum of stilbene in solution in the energy region 34 400 to 45 000 cm−1, which shows a strongly allowed two‐photon transition with a maximum at 41 000 cm−1. The upper state of this transition is of vibronic symmetry Ag. We believe that the electronic part of the upper state is also of Ag symmetry and that it is identifiable with the ’’phantom’’ singlet state postulated by Saltier and by Orlando and Siebrand as the major pathway for cis–trans photoisomerization of this molecule.

Two‐photon absorptivities of the all trans α, ω‐diphenylpolyenes from stilbene to diphenyloctatetraene via three wave mixing

Using the interference between the Raman resonances of the solvent and the two‐photon absorption resonances of the solute in a three wave mixing experiment (CARS), we have extracted the absolute two‐photon absorptivities at single energies for solution samples of the all ‐trans isomers of α, ω‐diphenyl‐ethylene, ‐butadiene, ‐hexatriene, and ‐octatetraene. Their absorptivities, for two identical linearly polarized photons in multiples of 10−50 cm4 sec photon−1 molecule−1 at the appropriate total energies (2?, cm−1), are as follows: stilbene, 12.1(0.9) at 38 873 cm−1, diphenylbutadiene, 14.4 (2.0) at 32 900 cm−1, diphenylhexatriene, 43.4 (4.0) at 32 900 cm−1, and diphenyloctatetraene, 61.0 (23.0) at 32 900 cm−1. Numbers in parentheses are standard deviations.

Polarized two‐photon studies of biphenyl and several derivatives

The two‐photon absorption spectrum of biphenyl dissolved in CCl4 is reported, using both the absolute two‐photon absorption technique and the two‐photon excited fluorescence technique. Polarization methods were used which permit the strong two‐photon absorptions to be assigned without resort to any subsidiary vibrational analysis. We find that the lowest singlet of biphenyl is a B state (in group D2, with twisted rings); its band stretches from about 33 000–36 000 cm−1, well to the red of the lowest strong one‐photon transition. We also find clear evidence of an A state hidden to one‐photon spectroscopy in the 42 000–45 000 cm−1 region. Other A states are also possibly present. Two‐photon spectra of 4,4′ dichlorobiphenyl, 4,4′ difluorobiphenyl, 2,2′ bipyridine, 2,2′ difluorobiphenyl, and terphenyl are also given and discussed.

Elastic light scattering by randomly oriented macromolecules: Computation of the complete set of observables

We present a calculation of all of the angle dependent observables that characterize completely the elastic scattering of light from solutions of macromolecules of arbitrary size and shape. The algorithm embodies the result of an exact analytic average over all orientations of the macromolecule, which may be large compared to the scattered wavelength. Our results are directly comparable to experiment; they are presented as a four‐by‐four Perrin matrix linking the four Stokes parameters of the incident light to the four Stokes parameters of the scattered light. The Perrin elements which we previously identified as retardation elements behave as they should: When we assume instantaneous electromagnetic interaction among the subunits of the macromolecule, they vanish; when the interaction is assumed to proceed at the speed of light, they are finite and may have easily measurable magnitude. Our algorithm captures all of the physical effects present in the Mie solution for scattering by spheres; but unlike the...

Dye laser two‐photon excitation study of several 0,0′‐bridged biphenyls

Polarized dye laser two‐photon excitation spectra are reported for the following series of 0,0′‐bridged biphenyls: fluorene, dibenzofuran, dibenzothiophene, carbazole, and phenanthrene. Two‐photon polarization selection rules are used to assign the peaks observed. Several new states are reported and a number of tenuous assignments in the literature are confirmed. The spectra of the bridged compounds are compared with the biphenyl two‐photon spectrum.

On the theory of polarized light scattering from dilute polymer solutions

A general microscopic theory of polarized light scattering from a dilute polymer solution is presented. Interactions between the subunits of the polymer are explicitly allowed, so that multiple scattering within the polymer is recognized. The incident and observed light may have any polarization. A formula for the light scattering from a collection of oriented polymer molecules is given, and then its orientation average is taken so that it applies to a dilute solution of the polymer. The averaging process results in a remarkable simplification of the scattering equation, terminating an infinite series exactly after five terms. The only remaining infinite series may be accurately truncated after two terms for polymers in the size range of biological interest, producing a closed expression for the scattered intensity. It is concluded that light scattering from biological polymers is governed by no more than eight important parameters (three zeroth order and five first order), even in the case of resonance R...

Toward a theory of the Perrin matrix for light scattering from dilute polymer solutions

We present a treatment of the polarization of light scattered by dilute polymer solutions. The treatment is complete in the sense that it gives a theoretical expression for every quantity that can be measured by the use of polarized light, at a given scattering angle and given optical frequency. The monomer units of the polymer are assumed to be small with respect to wavelength, but no such assumption is made for the polymer as a whole. The final closed expressions should be very accurate for bodies in the size range of enzyme molecules, and should permit calculations of light scattering properties as a function of polymer geometry.