Jan Kommandeur

The UV spectra and their calculation of TCNQ and its mono- and di-valent anion

Abstract The electronic absorption and emission spectra of TCNQ and its mono- and divalent anions were measured. An SCMO CI calculation in the PPP approximation was performed with one set of parameters for the three species yielding good agreement with experiment. The energy of the disproportionation reaction 2TCNQ− → TCNQ0 + TCNQ−2 was estimated at 3.5 eV.

ELECTRONIC RELAXATION OF BIACETYL IN VAPOR-PHASE

Abstract The emissions of biacetyl after pulsed dye-laser excitation were studied at pressures down to 0.05 mtorr. At all energies the time-resolved fluorescence was composed of a nanosecond and a microsecond component. At “zero” pressure the long lived phosphorescence was absent while the “hot” phosphorescence has the same time characteristics as the slow fluorescence. By increasing the pressure the slow fluorescence was quenched while the milisecond phosphorescence was induced. We determined the low-pressure emission characteristics and the pressure effects as a function of excitation energy. From our data we obtained the parameters describing the intermediate type singlet-triplet coupling, the radiative and non-radiative relaxation rates from the singlet and triplet levels and the cross sections for the slow fluorescence quenching, all as a function of energy. Strong evidence is obtained for the participation of rotational states in the intra-molecular relaxation. The important difference between the situation where the singlet levels are isolated (low energy) and where the singlet level widths overlap (at higher energies) is demonstrated. In the former situation very large fluorescence quenching cross sectios were found. It is further shown that for high energies at least two effective collisions are needed to obtain a thermalized triplet; the mean energy removed per effective collision is 2200 cm−1.

Spectrum of the molecular eigenstates of pyrazine

The molecular eigenstate spectrum belonging to the P and R branches of the1B3u(0-0) electronic transition of pyrazinc-h4 was recorded with a 200 kHz wide laser in a supersome nozzle at a temperature of ≈ 1 K. The square of the former transform fo the amplitude spectrum yields the quantum beats in the fluorescence decay that have been reported before.

Intermediate level structure, quantum beats, and nuclear spin effects in the electronic relaxation of the 1B3u (nπ*) state of pyrazine

The P(1) member of the rotational manifold of the 6a10 transition in the 1B3u (nπ) state of pyrazine shows intermediate level structure and quantum beats in its decay. The P(2) member already shows a fast component. The effect of nuclear spin on the relative magnitude of the fast component is shown by partial deuterium substitution.

New phase transitions in simple M-TCNQ-salts

Abstract Phase transitions were observed in a number of crystalline 1:1 TCNQ ion-radical salts by absolute paramagnetic susceptibility and electrical conductivity measurements over a wide temperature range. Heats of transition are reported. Susceptibility data are compared with quantitative predictions of current theoretical calculations.

Electron spin resonance of NO2

This study presents the results of an E.S.R. study of NO2 embedded in polycrystalline solid N2O4 at 77°k. The radicals were generated by u.v. photolysis of the host matrix. By proper choice of the experimental conditions, spectra can be obtained from which the same information about the NO2 molecule can be derived as from single crystal measurements, except for its spatial orientation(s) in the host matrix. The second-order solution of the hamiltonian describing the system is tractable for the relative spatial orientations and experimental results can be analysed easily. The magnetic parameters obtained from this second-order analysis are used for the calculation of the spin density distribution over the molecule, providing a check on existing electronic models for NO2. A comparison of these magnetic parameters with those of NO2 in other media shows that the interaction of the electron cloud of the radical with the environment depends on the type of host matrix that is used. Furthermore, the analysis of t...

Electronic relaxation of 1B3u(nπ*) pyrazine in a supersonic jet: Rotational state dependence of the nonradiative rate leading to biexponential decay

The decay of single rotational levels of the vibrationless first excited singlet state of pyrazine (1,4‐diazabenzene) following expansion in a helium supersonic jet and excitation with a pulse dye laser is biexponential, as it is for single vibronic level excitation in the bulk gas phase. However, both the relative contribution and the lifetime of the fast, ’’nonradiative’’, component are strongly dependent on the initial rotational state. A fit to the data shows that the rate of nonradiative decay is proportional to K2, where K is the projection of the rotational angular momentum vector on the (near‐symmetric) top axis. Both the interpretation of these results and their implications with respect to the theory of radiationless transitions are discussed briefly.(AIP)

Photoquenching: The dependence of the primary quantum yield of a monophotonic laser-induced photochemical process on the intensity and duration of the exciting pulse

Abstract Excited state absorption in large molecules leads to a decrease of the primary quantum yield of a photochemical or a photophysical process. Since then the quantum yield decreases with increasing light intensity this effect is called photoquenching. Kinetic analysis of the excitation in a general level scheme of a large molecule yields expressions for the quentum yield of a laser-induced photochemical process. Calculation of the quantum yield for various combinations of molecular parameters and laser pulse characteristics shows quenching of the photochemical process due to excited state absorption, at laser intensities for which bleaching effects and other nonlinear processes are negligeable. The applicability of the steady state approximation in analyzing laser-induced processes is discussed. Experiments are reported, which confirm the calculated intensity-dependent quantum yield function. Previous measurements of intensity-induced quenching can now be discussed quantitatively. Care should be taken in interpreting laser-induced photochemical yields, especially at mode locked laser intensities; correct values can only be obtained by extrapolation to zero laser intensity

High resolution lifetime measurements of the perturbed J’=0 levels of the 1B3u state of pyrazine

The lowest excited singlet 1B3u state of pyrazine is known to be coupled to a number of triplet 3B3u states. Using a strongly collimated molecular beam and a single frequency laser it is shown that the J’=0 of the 000 transition contains at least 36 states. We have individually excited eight of these states and studied its decay. The lifetimes found (typically 450 ns) do not scale with the intensities of the excitation spectrum. This deviation is caused by a nonradiative decay of the zero order 3B3u states. With a simple model it was possible to reconstruct the absorption spectrum, the energies of the zero order states and its coupling strengths. The zero order decay rates of the singlet and triplet states have been determined. The value obtained for the zero order singlet state is 5 MHz; the values for the triplet states range from 0.6 to about 5 MHz.

Molecules and ions in a donor acceptor complex

This paper reports the results of an x-ray analysis, electron spin resonance, electrical conductivity, u.v. and visible spectroscopic measurements on TMPD-chloranil. The results can be explained by assuming a molionic lattice, i.e. a crystal lattice built up of neutral TMPD and chloranil molecules together with bi-valent positive TMPD ions and bi-valent negative chloranil ions. Elementary excitations creating (chloranil- … TMPD++ … chloranil-) are presumed to be responsible for the measured paramagnetism. It was found that two modifications of TMPD-chloranil exist, which at room temperature are only distinguishable by the anisotropy of their E.S.R. linewidth. At low temperature both modifications suffer a phase transition, as observed on x-ray Weissenberg photographs and confirmed by E.S.R. measurements.