Thomas W. Scott

Perturbation selection rules for multiphoton electronic spectroscopy of neutral alternant hydrocarbonsa)

A one‐electron perturbation which is to significantly enhance a ‘‘forbidden’’ single‐photon or multiphoton optical transition between any two states of an alternant hydrocarbon must satisfy pseudoparity selection rules in addition to the usual group theoretical selection rules. Pseudoparity predicts whether vibronic or inductive perturbations, e.g., will be successful, while group theory specifies the proper perturbation symmetries. A principal result is that for a given electronic transition, the roles of vibronic, and inductive perturbations are complementary. Their roles reverse depending on whether the transition requires an even or odd number of photons.

Polarized two‐photon fluorescence excitation studies of pyrimidine

Two‐photon fluorescence excitation studies are reported for pyrimidine (1, 3‐diazine) dissolved in a hexane solution at room temperature. Quantitative two‐photon absorption strengths for the three lowest electronic absorption bands are measured relative to the benzene B2u ← A1g two‐photon transition. The B1(nπ*) ← A1 excitation profile matches the one‐photon absorption band closely, indicating an allowed two‐photon transition. However, the B2(ππ*) two‐photon intensity is vibronically induced by a 1600 cm−1 b2 vibration, even though transitions into this state are formally two‐photon allowed. In addition, polarization analysis is used to uncover a new transition in pyrimidine in a region of overlapping absorptions. This new transition is assigned to the lowest A2(nπ*) state. If this assignment is correct, the A2(nπ*), B1(nπ*) splitting is 1.4 eV. Spectroscopic CNDO and INDO calculations of the two‐photon absorption strengths are also reported.

Effects of perennial forage-legume living mulches on no-till winter wheat and rye

Abstract No-till winter cereals sown in narrow rows may compete successfully with perennial forage-legume living mulches that can fix nitrogen (N), conserve soil, increase dry-matter production, and suppress weeds. The effects of small-grain species, mulch species, and top-dress N on grain and mulch yield and the grain N concentration of winter cereals direct-drilled into legume living mulches were examined in a two-year field study in New York, U.S.A., on soils of the Lima and Kendaia series: fine-loamy, mixed, mesic Glossoboric and Aeric Hapludalfs. Wheat ( Triticum aestivum L.) or rye ( Secale cereale L.) were grown in monoculture or drilled into summer-established plots of alfalfa ( Medicago sativa L.), birdsfoot trefoil ( Lotus corniculatus L.), crownvetch ( Coronilla varia L.), ladino clover ( Trifolium repens L. forma lodigense Hort ex. Gams), red clover ( T. pratense L.), or white clover ( T. repens L.). Spring top-dress N was applied at 0 or 56 kg N ha −1 . Cereals were reseeded for a second season. Mulches generally interfered more and yielded more with wheat than with rye. Birdsfoot trefoil, crownvetch, and white clover had little effect on grain-yield the first year; birdsfoot trefoil and crownvetch interfered strongly with cereals the second year. Red clover did not affect rye grain-yield in the absence of top-dress N, but did tend to reduce wheat yield. Top-dress N increased cereal grain-yield and decreased mulch yield. In general, legume mulches did not appear to enhance cereal N nutrition the first year; red and white clovers appeared to contribute N to rye the second year. Second-year grain-yields were generally lower than first-year yields, due to increased interference from living mulches and broadleaf weeds. All living mulches except crownvetch suppressed weeds the second year. The results indicate that some species of perennial forage legumes may be suitable for use as living mulches for direct-drilled small grains, especially tall early winter cereals.

A Rydberg transition in benzene in the condensed phase: Two‐photon fluorescence excitation studies

The two‐photon states of benzene lying 6 to 7 eV above the ground state are probed by polarized two‐photon fluorescence excitation techniques. Dramatic changes in both the excitation spectrum and the polarization ratio are seen as the pure liquid is diluted in alkane solvents. A new two‐photon resonance, not seen in the pure liquid, appears in the spectrum of dilute benzene solutions. The polarization behavior of this new feature as well as its sensitivity to solvent and temperature is interpreted in terms of a two‐photon promotion of an electron from the e1g HOMO into the 3s Rydberg orbital of benzene in the condensed phase.

Multiphoton ionization of liquid benzene: the ionization mechanism☆

Abstract The multiphoton ionization of liquid benzene at 355 nm is found to be a three-photon process involving a two-photon produced intermediate state which lives 10 ns or longer. Polarization experiments can be rationalized if the two-photon absorption at 177 nm (6.99 eV) involves transitions into a mixture of final molecular states having A 1g , E 1g and/or E 2g symmetries.

Alternant hydrocarbon selection rules in the two-photon spectroscopy of perturbed benzene

Abstract Parisers (+,−) pseudo-parity labels for the electronic states of alternant hydrocarbons provide explicit selection rules governing perturbations of multiphoton forbidden optical transitions. One principal finding is that inductive substituent effects in benzene derivatives should significantly enhance the two-photon 1 L a transition, but ought to have little influence on the two-photon 1 L b transition. The two-photon fluorescence excitation spectra of benzene and monofluorobenzene are seen to confirm these predictions.

Two‐photon photoselection in rigid solutions: A study of the B2u←A1g transition in benzene

The first detailed study of simultaneous two‐photon photoselection is reported, using the benzene B2u←A1g two‐photon forbidden transition to illustrate this new technique. Polarized emission from the B2u fluorescent state is used to probe the symmetries of excited molecular states reached by two‐photon absorption in an initially isotropic solution. Three independent polarization measurements are reported for each of the e1u, e2u, and b2u false origins. These transitions span all possible D6h molecular symmetry species which are two‐photon allowed from the ground state in a single color experiment. It will be seen how the photoselection method complements the conventional circular/linear polarization measurements in fluid solutions by providing empirical symmetry assignments for the nontotally symmetric transitions. The partial randomization parameter e, commonly used in one‐photon photoselection studies to rationalize nonideal polarization values, is introduced into the two‐photon theory. This considerabl...

Electric field quenching of recombination fluorescence: a probe of two-photon ionization in the condensed phase☆

Abstract Electric field quenching of recombination flourescence following simultaneous two-photon ionization of TMPD in isooctaine is measured at pulse field strengths up to 4. 5 × 10 7 V/m. The different isoenergetic molecular states reached via one-photon and two-photon excitation yield similar values of the photophysical parameters which govern geminate charge pair formation and decay.

Electric fields in sandwich cells by electroabsorption and electroreflection; The Al/tetracene/nesatron cella)

Interfacial electric fields in photoactive Al/tetracene/indium oxide sandwich cells have been studied by electric field perturbed absorption and reflection spectroscopy. Theory shows how electroreflection relates to the electrostatic field at the very surface of tetracene, while electroabsorption depends on the intrinsic potential difference between each electrode/tetracene contact. The magnitude and orientation of each surface field and the interelectrode potential difference for the sandwich cell are reported. In addition, the spectral dependence of electroabsorption and electroreflection predicted by theory is shown to be in good agreement with experiment.

Influence of topsoil removal and fertilizer application on peanut yields from an Indonesian Ultisol

Experiments were conducted on a tropical Ultisol in west central Sumatra, Indonesia, to evaluate the effects of topsoil removal and the subsequent application of phosphorus and potassium fertilizers, and calcium lime, on crop productivity. Peanut (Arachis hypogaea L.) yields and total above-ground biomass production were determined in field plots and potted soils from which 0, 3, 7 and 15 cm of topsoil had been removed. The removal of 3 cm of topsoil, which was equivalent to the entire A-horizon resulted in a significant reduction in peanut yields and above-ground biomass in both field and potted soils. No significant yield differences in peanut yields or biomass were observed between] treatments from which ⩾3 cm of topsoil had been removed. The application of fertilizers and lime restored peanut yields to levels observed in unfertilized soils from which topsoil had not been removed. However, the use of off-farm inputs was economically unfeasible for most hillside farmers in the research area.