The two-photon phosphorescence excitation spectrum of triphenylene

The two-proton excited phosphorescence of triphenylene in PMMA matrix at 77 K was measured using a tunable flashlamp-pumped rhodamine dye laser in the effect spectral region 32000–36000 cm^?1. The band origin of the S₀S₂ transition, which is uncertain in the one-photon absorption spectrum, was observed at 33200 cm^?1. The vibronic features in the one-photon spectrum were reinterpreted.     

The two-photon excitation spectrum of pyrene

The two-photon excitation spectrum of pyrene and the polarization ratio Ω have been measured up to ≈2100 Å. Evidence of a strong vibronic couplin     

The two-photon spectrum of toluene vapor

The two-photon excitation spectrum of toluene-h₈ and toluene-d₈ vapor has been recorded under low resolution (1 cm^?1) in the region of the S₁ ← S₀ (¹B₂ ← ¹A₁) transition. Although the electronic transition is formally allowed in two-photon spectroscopy, a large fraction of intensity exists in a subsystem induced by the out-of-phase CC stretching vibration ν₁₄ (b₂). Band contours associated with each of the two assigned tensor components of the transition are identified and partially analyzed by comparison with the two-photon contours of fluorobenzene.     

The two-photon fluorescence excitation spectrum of dibenzothiophene

The two-photon fluorescence excitation spectra of dibenzothiophene crystal (≈77 K) and solution have been investigated in the spectral region 30000–38000 cm^?1 (≈3350-2600 Å). Two electronic transitions have been found. Both the oriented gas model results applied to the crystal and the circular/linear polarization ratio to the solution claim for an ¹A₁ ← ¹A₁ assignment of the transitions. Not completely resolved vibronic structures in the crystal spectrum were observed for both transitions for which a tentative assignment to A₁ total symmetry is discussed. It has been found that the intensity of the first electronic band system arises not only from a purely electronic (i.e., Franck-Condon) mechanism, but also from its interaction with a higher state through totally symmetric vibrations.     

Peculiarities in the low energy range of the bremsstrahlung spectrum

Peculiarities in bremsstrahlung and polarization bremsstrahlung from fast electrons caused by the collective contribution of target's atoms to the formation of emitted photons are considered theoretically. It is shown that such collective contribution can result in the strong suppression or, in the contrary, in substantial growth of the emission yield relative to that in the case of fast electron interaction with a separate atom. Some new effects of this kind are analyzed in detail and the possibility to observe them experimentally is estimated.     

A quantum-classical approach to the photoabsorption spectrum of pyrazine

We have used the time-dependent discrete variable representation (TDDVR) method to simulate the photoabsorption spectrum of pyrazine. The time-dependent molecular dynamics of pyrazine after excitation to the S2 electronic state is considered as a benchmark to investigate the S2 absorption spectrum. We have carried out the dynamics on a basic four-mode model of pyrazine with the inclusion of five major modes as well as the rest of the vibrational modes as bath modes. Investigations reveal the effect of bath modes such as energy and population transfer from the subsystem to the bath. Calculated results demonstrate excellent agreement with traditional quantum-mechanical findings during the entire propagation and converge to the exact quantum results when enough gridpoints are used. It appears that TDDVR, as a numerical quantum dynamics methodology, is a good compromise between accuracy and speed.     

The two-photon excitation spectrum of fluorene near 6 K

The two-photon fluorescence excitation spectrum of the fluorene single crystal at about 6 K is reported. Two k = 0 exciton bands arising from the p     

Vibronic activity in the two-photon spectrum of pyrene

A CNDO/S Cl calculation of the two-photon vibronic activity of some b_1u and b_2u vibrational modes of pyrene is presented. The two-photon amplitude tensor is discussed in terms of vibronic coupling coefficients calculated by means of the orbital-following method. The results are in good agreement with the experimental data taken from a recently observed two-photon spectrum of pyrene in a Shpolskii matrix. The role of the ground-state coupling with the first excited B_1u state has been investigated and is shown to be responsible for most of the vibronically induced two-photon intensity of the spectrum. The calculations also show that the 1310 cm^?1 (b_1u; observed ground-state value) mode is associated with the largest vibronic coupling coefficients and the strongest two-photon amplitude tensor and therefore must be correlated with the most intense B_1u X b_1u false origin ≈ 1496 cm^?1 from the pure (0-0) line.     

Vibronic activity in the two-photon spectrum of pyrimidine

The vibronic activity revealed by the TPE spectrum of pyrimidine has been examined and calculated in detail through a set of semi-empirical calculations. The results confirm previous interpretations of the experimental data and contribute to assign the symmetry of the lowest excited singlet states. Moreover, the most active modes have been individuated and the mechanisms of vibronic borrowing have been compared in the case of one-photon and two-photon spectra.     

Vibronic structure of the gas phase phosphorescence spectrum of p-benzoquinone

The phosphorescence spectrum of p-benzoquinone has been studied in the gas phase. The vibronic structure is analyzed in terms of frequencies of vibrations strong in the Raman spectrum. The most predominant progression vibrations are found to be two fundamentals of ν₃(CO) and ν₂(CC stretching) around 1680 cm⁻¹ in the Raman spectrum. These modes have not been observed separably in the phosphorescence spectrum of gaseous p-benzoquinone because of low resolution spectrographs employed in early work. Several vibrational frequencies in the excited ³A_u(nπ*) state are proposed on the basis of the analysis of several prominent sequence bands in the emission.     

The two-photon spectrum of liquid pyridine by thermal lensing techniques

Thermal lensing is exploited to investigate the electronic properties of pyridine. The two-photon spectrum and polarization ratio of liquid pyridine has been measured from 2900 to 2000 Å. Absorptions corresponding to the two lowest ππ* states, of benzene parentage, and to the second excited nπ* state have been assigned. The latter, of A₂ symmetry, borrows two-photon intensity from A₁ states through vibronic coupling induced by a₂ vibrations. Calculations of allowed contributions to the two-photon cross sections for the lowest four excited levels support the assignment.     

The applicability of the second-derivative method to room-temperature phosphorescence analysis

The use of the second-derivative (d²) technique to overcome some of the problems encountered in room-temperature phosphorescence analysis is investigated. Despite some reduction in the signal-to-noise values, the d²-technique was shown to be particularly suited to improving the selectivity of the assay. The utility of this method for applications in multicomponent mixture analysis is discussed. The two most significant figures of merit for room-temperature phosphorescence are the reduction of spectral overlap and the decrease of background interference.     

The whole-energy spectrum method in reconstruction of an infinite photon field with a complex energy spectrum

The method for unfolding the distribution of a radiation field with a complex energy spectrum from the whole-energy spectrum image is suggested. The reconstruction is applicable in situations where the relative contribution of energy fractions in the photon beams, incident in the scanning directions is known.     

The polarised two-photon excitation spectrum of benzene monocrystals at 6 eV

The two-photon excitation spectrum of a benzene single crystal at 4.2 K has been recorded in the region of the second absorption system. The onset of two-photon absorption occurs near 46 500 cm^?1 (quoted as a two-photon frequency). The spectrum has the appearance of a forbidden transition in that the system begins with weak lines which become dominated by an intense continuum at higher energies. The two-photon cross section at 55 000 cm^?1 (the limit reached in this study) is about 200 times greater than at 47 490 cm^?1 although the peak of this strongly allowed system has not yet been reached. The fwhm of the bands near 47 000 cm^?1 is 280 cm^?1, the same as in the one-photon spectrum at these energies. The polarisation ratio is much the same over the entire energy range, and is consistent with the two-photon operators (xz, yz) or (zz). An analysis of all the data available from the one- and two-photon spectra suggests that the transition is ¹B_1u ← ¹A_1g in which the vibronic intensity is derived from the ¹E_1u state in the one-photon and ¹E_1g in the two-photon spectrum.     

The pure rotational raman spectrum of pyrazine C4H4N2

The pure rotational Raman spectrum of pyrazine, C₄H₄N₂, has been observed for the first time. The spectrum was recorded photographically using a 3 m spectrograph with a reciprocal linear dispersion of 1.4 cm⁻¹ mm⁻¹. The following rotational constants were obtained:

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Analysis of vibronic intensities in the phosphorescence spectrum of dimethylbenzaldehydes in durene

An attempt is reported to explain the main intensity patterns in the phosphorescence spectra of 2,4-, 2,5- and 3,4-dimethyl-benzaldehyde-1h₁ and -1d₁, observed previously. The analysis is based on CNDO and MINDO calculations of (transition) dipole moments, spin-orbit couplings, vibronic couplings, state energies, normal coordinates and vibrational frequencies. Where possible these quantities are empirically checked and corrected. Additional information, especially about the separation of the closely spaced T₁(³ππ^*) and T₂(³nπ^*) states, is obtained from phosphorescence excitation spectra reported here for all six isomers. The phosphorescence spectra consist of two components, an “allowed” component of ³ππ^* and a “forbidden” component of ³nπ^* symmetry. It is concluded that the allowed component is partly induced by the crystal field. The forbidden component is vibronically induced by out-of-plane vibrations among which the aldehydic CH(CD)-wag mode is the most active. The observed intensity patterns for this component are ascribed to interference between two mechanisms, one involving vibronic coupling between S₀ and S₁(¹nπ^*) and spin-orbit coupling between S₁ and T₁, the other involving vibronic coupling between T₁ and T₂ and spin-orbit coupling between S₀ and T₂. Within the groups of either 1h₁ or 1d₁ isomers, the main changes in the spectrum are shown to be due to the change in T₁–T₂ energy separation. The changes observed upon deuterium substitution in the aldehyde group involve, in addition to changes in the T₁–T₂ gap, changes in vibronic coupling due to normal-coordinate mixing. All these spectral changes are reproduced by calculations based on a mixture of theoretical and empirical input parameters, derived from, or at least consistent with, other observations, including excitation spectra, dipole moments and zero-field splittings. It is concluded that the mechanisms underlying these calculations offer a satisfactory explanation of the observed intensity patterns in the phosphorescence spectra of dimethylbenzaldehydes.     

The two-photon excitation spectrum of triphenylene in n-heptane single crystals

A comparison of the one-and two-photon absorption spectra of triphenylene in n-heptane single crystals at 1.6. K reveals that the lowest singlet state (S₁) is of ¹A′₁ symmetry while the second singlet state(S₂) has ¹A′₂ symmetry, in contradiction with the commonly accepted assignment.     

The two-photon excitation spectrum of a carbazole single crystal at 4.2 K

The low-energy system in the polarized two-photon excitation spectrum of carbazole at 4.2 K has been measured by detecting fluorescence pulses from deliberately added anthracene. Herzberg—Teller activity was detected for Some A₁ vibrations. The two-photon operators contribute to the two-photon cross section in the order yy ≈ zz ? xx.     

The two-photon excitation spectrum of a dibenzofuran single crystal at 4.2 K

The polarised two-photon excitation spectrum of crystalline dibenzofuran at 4.2 K has been measured. Only totally-symmetric vibrations are active, and their relative intensities are similar to those in the one-photon spectrum already reported. No factor-group splittings are found. The origin in the two-photon spectrum has a width of ? 10 cm^-1 in contrast to a width of ? 250 cm^-1 in the one-photon spectrum, now attributed to extreme inhomogeneous broadening.