A.K. Sharma

Study of photophysical properties of Coumarins: substituent and concentration dependence

Abstract The results of the gain spectra studied for six Coumarins with different substituents at the 4th and 7th positions are reported. These dye concentration dependent gain spectra have been studied by the amplified spontaneous emission (ASE) technique using an N 2 laser (337.1 nm) in the concentration range 1 × 10 −5 to 1 × 10 −2 M. The dyes are so selected that a comparative study could also be possible between rigid and nonrigid structures. All studies are made with a polar solvent. The observation of two bands in the gain spectra of nonrigid coumarin dyes in polar solvent (ethanol) is of great interest and the anomalous long wavelength peak formation is attributed to twisted intramolecular charge transfer (TICT) state formation in the excited state. The dominant stabilization of the TICT state is due to a long range specific polarization interaction with polar solvent molecules. The nonradiative decay path (ICT → TICT transition) is found to be dependent on the Coumarin structure and substituent pattern. The relationship between the short wavelength bands is also explained. The dependence of lasing wavelength and the peak gain on the dye concentration have also been correlated with the variation of fluorescence life time which is due to radiation trapping and concentration quenching.

Study of laser behaviour in C-440 and C-485 dye mixture

Abstract The laser behaviour in the C-440 (donor) and C-485 (acceptor) dye mixture in ethanol is studied and discussed by investigating the gain characteristics at various acceptor concentrations (1 X 10 -4 M/L to 1 X 10 -2 M/L) and pump powers of the N 2 - laser. It is found that most of the pump power absorbed by C-440 is transferred to C-485 as a useful pump power. The gain characteristics of C-485 are found changed due to the change in the effective fluorescence lifetime caused by the energy transfer reaction. The theoretical calculations were also done to find the total transfer efficiency at various acceptor concentrations to identify the appropriate energy transfer mechanism responsible for gain enhancement in C-485. These calculations take into account the contributions due to both radiative and nonradiative energy transfer. The energy transfer rate constants and critical transfer radius ( R 0 ) are calculated by using Stern-Volmer plots and concentration dependence of radiative and nonradiative transfer efficiencies. The concentration dependence of laser peak gain and the lasing wavelengths of the ETDL have also been studied. The experimental results indicate that the dominant mechanism responsible for the efficient excitation transfer in this mixture is of nonradiative nature due to long range dipole-dipole interaction. It is also found that the TICT state formation possibility in excited states is either reduced for the acceptor dye in the presence of donor or these TICT states become nonradiative.

Concentration-dependent optical gain characteristics of Coumarin 485, Coumarin 440 and Rhodamine B

Abstract The dye-concentration dependent gain spectra for Rhodamine B (Rh B), Coumarin 485 (C 485) and Coumarin 440 (C 440) in ethanol have been studied by the technique of amplified spontaneous emission (ASE) under N2 laser (337.1 nm) excitation in the concentration range 1 X 10-5 to 1 X 10-2 M. The dependence of the lasing wavelength and peak gain on concentration have been understood in terms of the variation of fluorescence lifetime, which is due to photophysical processes such as radiation trapping and concentration quenching. A comparison of the dynamics of the internal degrees of freedom in C 485 and C 440 has also been made on the basis of different functional groups at different positions of the basic Coumarin.

Multiphoton photoconductivity in an indirect band gap crystal: CdI2

Abstract The photoconducting properties of CdI2, an indirect band gap crystal, have been studied using fundamental and frequency doubled Nd : YAG laser. The ration of three and two photon absorption cross sections ( v 3 v 2 ) has been calculated with the help of experimental results. The value of two photon absorption cross section v2 has been calculated by using third order time dependent perturbation theory and that of v3 has been estimated by making use of the experimental results. Low values of the absorption cross sections obtained in the present case have been attributed to phonon assisted transitions.

Photoquenching in laser grade dyes: Part I

Abstract Optical gain measurements have been made in Rhodamine-B (Rh-B) and Coumarin-495 (C-495) ethanolic solutions at different wavelengths, pump intensities, concentrations and cell length using the amplified spontaneous emission (ASE) technique. A comparison of the experimental results with the theoretical predictions indicates that photoquenching dominates in the case of C-495, while some other processes like dimer formation, intersystem crossing or two-photon excitation may be dominating in the case of Rh-B.

Time-resolved spectra of coumarin 30-rhodamine 6G dye mixture

The effect of acceptor concentration on the energy transfer from Coumarin 30 (donor) to Rhodamine 6G (acceptor) has been studied. The nature of energy transfer reaction has been studied through lifetime measurements by recording the time-resolved fluorescence decay curves. The energy transfer parameters calculated were used to confirm the occurrence of energy transfer on the basis of the emission-reabsorption effect.

Concentration and pump intensity dependent gain studies for disodium flourescein (FDS), cresyl violet (CV), and rhodamine- 590 (Cl) and cresyl violet mixture

Abstract The dye concentration dependence gain spectra for disodium fluorescein (FDS) cresyl violet (CV) and a dye mixture [Rh 590 (Cl)+CV] dissolved in methanol have been studied by amplified spontaneous emission (ASE) technique under pulsed N2-laser excitation. On comparing the efficiency curves, the dominant role of photoquenching (excited state absorption) was clearly observed in the non-rigid dye FDS, whereas it is non-existent in rigid dye CV at the same concentration. In case of a dye mixture, the energy transfer excitation was found to overcome the inner filter effect and other losses. The various energy transfer mechanisms in these dyes have also been discussed.

Energy Transfer Study on Coumarin-30-Rhodamine-6G Dye Mixture Using a Laser Fluorimeter

Abstract A study has been made of the effect of acceptor concentration on the energy transfer from Coumarin-30 (donor) to Rhodamine-6G (acceptor) using a laser fluorimeter. The nature of the energy transfer reaction has been studied through life time measurements by recording the time-resolved fluorescence decay curves. The energy transfer parameters calculated were used to confirm the occurrence of energy transfer on the basis of the emission-reabsorption effect.

A PULSED N2-LASER STUDY OF THE EFFECTS OF SUBSTITUENTS ON THE EFFICIENCY OF COUMARINS

Abstract Efficiency curves for six Coumarins with different substitutions at 4th and 7th positions are reported. Substituents cause a change in excited-state dipole moment for the fluorescent states, in restrictions of rotatory motion for the amine group at the 7th position, and the delocalisation of excitation energy away from the Coumarin moity. The dominant photophysical features for Coumarin dyes are discussed in terms of emission from an intramolecular charge transfer (ICT) excited state and an important non-radiative decay path involving the rotation of the amine functionality (7th position) leading to a twisted intramolecular CT state. This non-radiative decay path is a function of Coumarin structure, as well as some other factors, such as solvent polarity and temperature, etc. Out of all these factors most sensitive is substituent pattern for a given polar solvent. The effect of these substituent groups at different positions is discussed.

Pulsed laser induced photoconductivity in II–VI crystal : ZnSe

Abstract The photoconductivity properties of ZnSe crystal have been studied using Nd: YAG and N 2 -laser excitation. The charge vs intensity characteristics (Q vs I) at low photon densities indicate non-linear behaviour in both the cases. Very low values of the electron mobility at small applied voltage and low photon density with a phenomenal increase with field have been detected.