Karel E. Drabe

Localization of spontaneous emission in front of a mirror

Abstract We show that the fluorescence emitted in front of a mirror exhibits angular and spectral localization effects that are caused by self-interference in the spontaneous emission from a Wiener-fringe pattern. A semi-classical description is given and found to be in good agreement with the observations.

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.

Fluorescence properties of submonolayers of rhodamine 6G in front of a mirror

Fluorescence properties of submonolayers of rhodamine 6G have been measured as a function of distance to an aluminum mirror. For distances less than 5 nm (spacer layers of 1 nm thickness were used) a broadening of the emission spectrum is observed. The fluorescence lifetime has been measured for distances up to 6 nm. The broadening of the fluorescence spectra and the shortening of the lifetime observed when the sample is close to the mirror are attributed to efficient energy transfer from the monolayer to the mirror. The experiments clearly demonstrate that both volume and surface contributions must be considered in this energy-transfer process.

Nonlinear optical properties of Langmuir–Blodgett monolayers: Local‐field effects

Detailed measurements of the macroscopic second‐order optical nonlinearity χ(2)(2ω,ω,ω) of Langmuir–Blodgett dye‐doped monolayers are reported. The observed deviations from a linear behavior of χ(2) with increasing surface density are shown to be due to local‐field effects. In order to calculate these local‐field factors for disordered systems, a novel Monte Carlo type calculation is introduced. This calculation not only accounts for density variations in the monolayers but also incorporates the effect of off‐diagonal elements of the (microscopic) linear susceptibility tensor. Quantitative agreement is found between the calculations and the experimental results using only the molecular hyperpolarizability as a free parameter. A method is presented to determine the tilt angle of the chromophores in Langmuir–Blodgett monolayers from the anisotropy of the linear absorption. The tilt angle determined this way is in excellent agreement with a determination by second‐harmonic generation.

Assignment and Analysis of the Absorption and Emission Spectra of the Lowest nπ* Triplet State in 9,10-Anthraquinone

Abstract Polarized Zeeman absorption experiments on 9,10-anthraquinone crystals show the lowest triplet state in this molecule to be a g nπ* state. The gap between this state and the higher u nπ* state is found to be 410 cm−1. The phosphorescence spectrum of an isotopically mixed crystal of AQ-h8 in AQ-d8 is analyzed in detail and confirms the orbitally forbidden nature of the emitting state. The results are compared with those previously obtained for p-benzoquinone.

Non-Resonant Light Scattering in Pyrazine and Its Quantum Yield

The ratio of the fast to the slow component in the decay of the 1B3u state of pyrazine is shown to vary strongly across the rotational manifold. In P(1) the fast component is absent, between the transitions it is dominant. Therefore, the excitation spectrum depends on the time the fluorescence is registered. The effects are due to non-resonant light scattering (NRLS), which because of its high quantum yield, shows up as unexpectedly strong. NRLS contributes to the fast component.

(2+1) REMPI PES of Gerade Rydberg states of molecular bromine in the 68000 to 73400 cm−1 region

Abstract Five vibrational progressions associated with transitions to Rydberg states in molecular bromine are reported in the region 68000 to 73400 cm−1 (8.5 to 9.1 eV). These Rydberg states are accessed by two-photon absorption, and subsequently ionised by a photon of the same frequency. The photoelectron spectra of these states reveal unperturbed Rydberg character, and unambiguously allow the vibrational assignment of the Rydberg progressions and the determination of the convergence limit (i.e. the core) of these Rydberg states. The values obtained for the origin and the cores of the Rydberg states are 68825 cm−1 (2Πg 3 2 ), 70568 cm−1 (2Πg 3 2 ), 71750 cm−1 (2Πg 1 2 ), 72440 cm−1 (2Πg 3 2 , 72904 cm−1 (2Πg 3 2 ). Evidence for perturbations in a few vibrationally excited Rydberg states is found in the photoelectron spectra.

The absolute value of the quantum yield of the fluorescence of the 1B3u 0–0 state of pyrazine as a function of the rotational quantum numbers

In this paper we fit low‐resolution spectra of pyrazine by assuming Coriolis coupling between S1 and {S0}. Evidence for Coriolis coupling is particularly obvious in the rovibronic spectra of pyrazine‐d3h1 of which we give high resolution examples. For the lowest rotational temperature we noticed a non‐Boltzmann distribution of the J‘=0 ground state, which is probably caused by a bottleneck for ΔJ‘=−2 transitions. Using the lifetimes of molecular eigenstates belonging to P(1) we can calculate the absolute quantum yield of the vibrationless and rotationless 1B3u state. Using this number and the obtained interstate Coriolis coupling rate constants we calculate the variation of the absolute quantum yield across the rotational contour. For low J we get satisfactory agreement with experiments, but for high J our calculations drop off too fast with J. We explain this by the fact that at high J values triplet decay becomes dominant because of K scrambling in the triplet manifold.

Photoelectron spectroscopic study of resonant multiphoton ionisation of atomic and molecular bromine

Abstract A photoelectron spectroscopic study of resonance-enhanced multiphoton ionisation (REMPI) of bromine in the visible region from 430 to 510 nm has been performed. Photoelectron spectra of several resonances originating from (3+2) and (4+1) REMPI of ground-state and spin-orbit excited-state atomic bromine are obtained and analysed. The photoelectron spectra obtained from (3+1) REMPI of Br2 show qualitatively simple Franck-Condon progressions and core-selective ionisation to the 2Πg, 1 2 excited ionic state. The spectra confirm a previous tentative vibrational assignment of the band system. However, the present results indicate an intermediate electronic state which appears not to be in agreement with earlier single-photon absorption measurements.

Magnetic field dependence of rotationally resolved excitation spectra of the 1B3u 000 transition of jet-cooled pyrazine

Abstract We report rotationally resolved excitation spectra of the 1B3u O00 transition of jet-cooled pyrazine in magnetic fields up to 50 kG. The emission intensity of every rotational line is found to decrease by a factor of three for magnetic fields larger than about 300 G. For still larger magnetic fields up to 50 kG the total emission intensity remains constant. The effect of collisions on the threefold quenching of the emission is studied separately, and found to be of minor importance. It is concluded that the reduction of the quantum yield due to the magnetic field originates from an intramolecular process. The threefold decrease of the quantum yield is interpreted in terms of a nonradiative decay rate constant of the zero-order triplet states. The decay rate constant consists of both a magnetic field dependent part (about 0.3 MHz at zero field) and a magnetic insensitive part (about 0.7 MHz).