Nanosecond Pulse Excitation Research Articles

Amine-assisted photochemical dehalogenation of haloanthracenes (9-halo and 9,10-dihalo compounds) and their triplet formation

Steady-state photolysis of haloanthracenes (XA; 9,10-dichloro, 9,10-dibromo, 9-chloro and 9-bromo compounds) in acetonitrile-amine (triethylamine or N,N-dimethylaniline) causes the consecutive reactions; 9,10-dihalo compounds→9-halo compounds→ anthracene. Although both the lowest excited singlet [1XA(S1)] and triplet [3XA(T1)] states of XA are quenched by amine, the appearance of absorption spectra due to the haloanthracene radical anions (XA-) within the duration of nanosecond pulse excitation indicates that the intermediates for dehalogenation of XA are XA- produced by a diffusion-controlled reaction of1XA(S1) with the ground-state amine yielding the singlet exciplexes [1(XA-amine)*] which decompose rapidly into XA- and the amine radical cations. Moreover, an amine-assisted formation of3XA(T1) has been attributed to the intersystem crossing from1(XA-amine)* to the triplet exciplexes [3(XA-amine)*] followed by decomposition into3XA(T1) and ground-state amine.

Reinvestigation of photochemical debromination of 9,10-dibromoanthracene and 9-bromoanthracene by amine (triethylamine or N,N-dimethylaniline) studied by nanosecond laser photolysis in acetonitrile at room temperature

Abstract From measurements of the fluorescence and triplet—triplet absorption spectra of bromoanthracenes (XA; the 9, 10-dibromo and 9-bromo compounds) in acetonitrile—amine (triethylamine or N,N -dimethylaniline) at room temperature, the lowest excited singlet states [ 1 XA(S 1 )] of XA are found to be quenched by amine with rate constants of the order for that of a diffusion-controlled reaction. Although the lowest excited triplet states [ 3 XA(T 1 )] of XA are also quenched by amine, the triplet quenching rate constants (4.9 × 10 4 -1.1 × 10 5 dm 3 mol −1 s −1 ) are much smaller than those (1.6 × 10 9 –6.2 × 10 9 dm 3 mol −1 s −1 ) obtained with azulene and ferrocene. Since the decrement in reactant absorptions during steady-state photolysis is not affected upon addition of azulene and ferrocene (1 × 10 −4 mol dm −3 ), the appearance of absorption spectra due to the bromoanthracene radical anions (XA .− ) within the duration of nanosecond pulse excitation and their decay rate constants of 2.4 × 10 5 -1.8 × 10 6 s −1 indicate that the intermediates for debromination are XA .− produced by a diffusion-controlled reaction of 1 XA(S 1 ) with ground-state amine yielding the singlet exciplexes [ 1 (XA—amine)*] followed by the decomposition into XA .− and the amine radical cations. Although 1 (XA—amine)* intersystem cross to the triplet exciplexes [ 3 (XA—amine)*] giving rise to an additional formation of 3 XA(T 1 ), no evidence supporting the participation of 3 (XA—amine)* in the debromination as proposed by Soloveichik et al., High Energy Chem., 23 (1989) 281, translated from Khim. Vys. Energii, 23 (1989) 351 is obtained.

Emission of fluorescence from chlorophylla in vivodue to nanosecond pulsed laser excitation

A model was proposed and tested experimentally to describe the emission of fluorescence by chlorophyll a in vivo as a result of pulsed laser excitation. This model takes into account the migration of excitons between various photosynthetic units, singlet-singlet annihilation of excitons, pigment bleaching, and also the influence of various states of the photosystem II reaction centers. A method was developed to measure the average number of excitons reaching a photosystem II reaction center during a pulse. This involved two-pulse laser excitation. It was found that the rates of exciton capture by the reaction centers were the same for the PIQ and P +IQ − states of the photosystem II reaction centers, whereas the rate of exciton capture in the P +I −Q − state was half that for the PIQ − state.

Lifetime measurement of the T=23 537 cm-1, J=9/2 odd-parity level in Nd II using time-resolved collinear fast-beam laser spectroscopy.

Time-resolved collinear fast-beam laser spectroscopy has been used to measure the lifetime of the T=23 537 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, J=9/2 odd-parity [abbreviated as (23 537)${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{9/2}$] level in Nd ii. Nanosecond pulsed excitation was achieved with the Doppler switching technique. The fast ions were selectively excited from the metastable 4${\mathit{f}}^{4}$5d $^{6}$${\mathit{K}}_{9/2}$ level to the (23 537)${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{9/2}$ level by the light of a cw dye laser. The decay of the resonance fluorescence photons emitted in the transition to the ground 4${\mathit{f}}^{4}$6s $^{6}$${\mathit{I}}_{7/2}$ level, as a function of the flight distance, was observed. The decay length was scaled into the decay time by the ion velocity, which was determined using the optical resonance itself. The lifetime is 27.7(9) ns.

Spectroscopic investigation on the many-body effects under stationary and quasi stationary optical pumping in GaAs/AlGaAs superlattices

We report a spectroscopic investigation on the many-body effects induced by intense optical pumping in GaAs/AlGaAs superlattices. Under nanosecond pulsed excitation band filling effect and stimulated emission due to the photogenerated electron-hole plasma are observed. Evidence of a new emission band probably due to the Γ - X mixing of the electronic wavefunctions in the superlattice, is reported. Under continuous wave optical pumping the biexciton luminescence is observed. The exciton-biexciton optical conversion process has been also observed by means of a continous wave pump and probe technique, employing the sample reflectance like a probe.

Fluorescence lifetime studies of no a 2Σ +(ν = 5, N = 9), B 2Π [formula omitted](ν = 8, J = 8.5), C wΠ [formula omitted](ν = 1, J = 8.5), D 2Σ + (ν = 0, N = 5) and D 2Σ +(ν = 1, N = 9)

Fluorescence decay profiles of NO excited levels slightly above the dissociation limit have been measured by a single-photon counting technique with nanosecond pulse excitation using an iodine flash lamp. Three iodine atomic lines in the vicinity of 180 nm are found to bring NO molecules into the levels A 2Σ +(ν = 5, N = 9), B 2 2Π 3 2 (ν = 8, J = 8.5), C 2Π 3 2 (ν = 1, J = 8.5), D 2Σ +(ν = 0, N = 5) and D 2Σ +(ν = 1, N = 9). Extrapolated zero-pressure lifetimes for single rotational levels are obtained, except for the C state where only a lifetime of ⩽0.4 ns was obtained. Self-quenching rate constants are also determined under higher-pressure conditions. Helium was found to quench the NO A 2Σ +(ν′ = 5) fluorescence very efficiently.

Time‐Resolved Degenerate Four‐Wave Mixing in CuCl under Nanosecond Pulsed Excitation

AbstractUsing nanosecond laser pulses the time behaviour is investigated of degenerate four‐wave mixing close to the biexcitonic resonance in CuCl. The signal emission presents different time evolutions depending on excitation conditions. Its generation rate shows intensity changes and retardation effects. These effects are explained by an attenuation of the pump pulse through a time‐dependent nonlinear absorption and, at the resonance, by a loss of coherence related to collisions with bi‐excitons.

Effective overcoming of frequency detuning and resonance effects under molecular vibrational excitation by picosecond infrared laser pulses

A number of frequency detunings, in particular those caused by vibrational anharmonicity, can be compensated under coherent excitation of molecules by picosecond infrared laser pulses. Multi-photon excitation by a sequence of picosecond pulses is much more effective than nanosecond pulse excitation with the same energy fluence.

Luminescence of High Density Electron-Hole Plasma in GaAs

Spontaneous luminescence and optical gain of high density electron-hole phasma (EHP) in GaAs are observed at 4.2 K under nanosecond pulse excitation. Time-resolved luminescence spectra are observed under picosecond pulse excitation. The chemical potential and reduced band-gap energy of the EHP are obtained from the spectral shape analysis. These values and also their time changes are explained qualitatively by the calculation of the EHP ground-state energy. After picosecond pulse excitation, hot carriers cool down rapidly to 50 K within the first 150 ps, but the thermal equilibrium with the lattice is not attained up to 400 ps. The observed gaseous EHP is considered to exist only in a transient state and the electron-hole liquid state is not realized.

Fluorescence and Photosynthesis: Gated Detection and Analysis of Nanosecond Pulse Excitation

Fluorescence and Photosynthesis: Gated Detection and Analysis of Nanosecond Pulse Excitation

Calculation of two-photon conductivity under nanosecond and picosecond pulse excitation in semiconductors

It has been the usual practice to solve the continuity equation for the case of steady excitation and presume that the same results are valid for pulse excitation of the photoexcited carriers. However, it has been shown in the present analysis that this assumption is valid for the exciting pulse which is of longer duration than the mean lifetime τ of the photoexcited carriers. In the case of excitation of photoconductivity by picosecond pulses, which are now available from the mode-locked laser system, the pulse duration is much smaller than τ and the results of steady-state excitation are no longer applicable. Moreover, the magnitude of the photoconductivity depends on the pulse duration T and is independent of τ. In fact, the photoconductivity is found to exhibit a transient behavior.

Optical formation and luminescence of excitonic molecule in CdS

The induced absorption due to the optical formation of an excitonic molecules from a single exciton, i.e. to the reverse process of excitonic molecule luminescence, is observed for CdS. Luminescence spectra of excitonic molecules under picosecond pulse excitation with mode-locked lasers are measured for CdS and CdSe. Notable differences are observed between spectra under picosecond and nanosecond pulse excitations and also between spectra excited by one-photon and two-photon absorptions.