Narrow-band Laser Excitation Research Articles

Luminescent solar concentrators 2: Experimental and theoretical analysis of their possible efficiencies

Experimental techniques are developed to determine the applicability of a particular luminescing center for use in a luminescent solar concentrator (LSC). The relevant steady-state characteristics of eighteen common organic laser dyes are given. The relative spectral homogeneity of such dyes are shown to depend upon the surrounding material using narrowband laser excitation. We developed three independent techniques for measuring self-absorption rates; these are time-resolved emission, steady-state polarization anisotropy, and spectral convolution. Preliminary dye degradation and prototype efficiency measurements are included. Finally, we give simple relationships relating the efficiency and gain of an LSC to key spectroscopic parameters of its constituents.

Nonsteady-state molecular luminescence behavior under narrowband laser excitation

The time behavior of a two- and a three-level molecular system under narrowband pulsed-laser excitation is discussed, and relations are presented for the temporal variation of the concentrations of the levels involved. It is shown that if saturation is approached, the response time of the molecular system is strictly related to the pumping time, which is given by the reciprocal of the product of the laser spectral density and the Einstein coefficient of stimulated absorption. Typical calculations of the fractional number densities of the levels are presented as a function of the value of the laser spectral density required to achieve saturation for a few typical molecules such as benzophenone, fluorene, and rhodamine 6G.

Fluorescence spectra of laser-excited YO molecules in a H 2O 2Ar flame

CW dye laser induced fluorescence emission and thermal emission spectra of YO-molecules in a 1 atm H 2O 2Ar flame of 2430 K were recorded simultaneously. Narrow band laser excitation was applied to four rotational lines in the (1, 1) Q-branch of the A 2Π 3 2 → X 2Σ + transition and broadband excitation was applied to several separate Q-branches of the A 2Π 1 2 , 3 2 → X 2Σ + transitions. From the differences between the fluorescence emission spectra and thermal emission spectra, we conclude that collisional de-excitation of an excited vibronic level takes place by vibrational relaxation, decay to the electronic ground state and by intermultiplet transfer in order of increasing probability.

Optical free induction and fluorescence decay of pentacene in p-terphenyl following narrow band laser excitation

Optical free induction and fluorescence decay experiments on pentacene following narrow band laser excitation are reported. From the results we conclude that pentacene for its lowest 1B 2u ← 1A 1g transition is a perfect two-level system and from the power saturation term in the OFID we calculate a transition dipole moment of 1.5 ± 0.2 debye. A long-lived transient observed in the fluorescence during the onset of cw dye laser excitation is ascribed to a bottle-neck in the optical pumping cycle.

Narrow band laser excited fluorescence as a probe of the near-resonance vibronic coupling in isoquinoline vapor

Resonance fluorescence from isoquinoline vapor has been generated by using tuned, narrow-band (0.2 cm −1 ) laser excitation to pump each of eight bands in the 3100 A absorption system. The fluorescence spectra, in contrast with the complex vapor absorption spectrum, consist of discrete, open vibrational structure which is analyzed readily. The fluorescence confirms that the 3100 A transition derives from the near-resonance vibronic coupling between two close-lying electronic states: 1 A″(nπ * ); 1 A′(ππ * ). The vibronically tangled spectrum can be understood in terms of a model which emphasizes nearresonance vibronic interactions but also includes certain off-resonance contributions. The fluorescence spectra serve to identify the dominant off-resonance contributions. Fluorescence intensities calculated theoretically are found to match intensities observed in fluorescence from two selectively excited levels, denoted 0 + and 0 − . These two levels are reached via laser excitation of two prominent bands at 31925 and 31983 cm −1 which, together with a third weaker band at 32008 cm −1 , are shown to comprise the electronic origin of the 3100 A absorption system. Fluorescence from each of the eight levels identified their dominant vibronic parentage. Three other less distinct, higher energy absorption features have also been probed with laser excitation. In these cases, fluorescence is broad and congested, indicating that higher vibrational levels in the 1 nπ * − 1 ππ * interaction manifold are severely mixed.