Thermal Lensing Experiment Research Articles

Spectral and Non Radiative Decay Studies of Lead Di Bromide Single Crystals by Mode Matched Thermal Lens Technique.

In the present paper, the investigations on the non radiative decay mechanism, optical band gap determination from absorption spectroscopic studies and fluorescence emission by photo luminescence techniques using different excitation wavelengths on gel derived lead di bromide single crystals are reported. Non radiative decay of the sample is studied using high sensitive dual beam mode matched thermal lens technique. For the thermal lensing experiment the crystal in solution phase is incorporated with rhodamine 6G dye for enhancing the absorption of the crystal sample. The thermal diffusivity of lead di bromide is determined using the probe beam intensity v/s time measurements.

Photophysical properties of dioctyl 4-methoxybenzylidenemalonate: UV-B absorber

Dioctyl 4-methoxybenzylidenemalonate (DOMBM) is an effective stabilizer for photolabile 4-tert-butyl-4'-methoxydibenzoylmethane (BMDBM). DOMBM is superior to the most widely used UV-B absorber, octyl methoxycinnamate (OMC), at preserving its UV-B absorbance in the presence of BMDBM. The energy levels of the lowest excited singlet (S(1)) and triplet (T(1)) states of DOMBM were determined by measuring fluorescence and phosphorescence spectra in ethanol at 77 K. The energy level of the T(1) state of DOMBM is lower than that of BMDBM and is higher than that of OMC. The triplet-triplet energy transfer from BMDBM to DOMBM was demonstrated by measuring the electron paramagnetic resonance (EPR) and time-resolved phosphorescence spectra. The phosphorescence and EPR signals of DOMBM are too weak to be observed through the direct excitation in ethanol at 77 K. The phosphorescence spectrum was observed by using external heavy atom effects of ethyl iodide. The EPR spectrum was observed by using benzophenone as a triplet sensitizer. The fluorescence quantum yield of DOMBM is small in ethanol at room temperature. Only the fast component, due to the heat released from the excited state of DOMBM, was observed in the time-resolved thermal lensing experiments at room temperature. These results show that the quantum yield of the S(1) → T(1) intersystem crossing is negligible and the deactivation process is predominantly internal conversion to the ground state for DOMBM.

Measurement of the two-photon absorption cross section by means of femtosecond thermal lensing

We present a variation of the single-beam thermal lensing experiment to determine the two-photon absorption cross sections of classical fluorophores. The approach is based on comparison of two thermal lensing signals simultaneously induced by a one- and two-photon absorption process from a high-repetition-rate femtosecond laser system. As a consequence of this comparison, a simplified expression independent of the several experimental parameters is obtained. Additionally, because of the low incident power levels required, undesirable optical effects such as Kerr or Raman scattering are avoided. Our experimental results agree well with those recently published for luminescent methods, validating the approach.

Thermal Lensing in a Supercritical Water Medium

A pulsed, two-beam, thermal lensing experiment was performed to determine the concentration of aqueous solutes above the critical point of water. Despite a very significant mirage effect due to thermal gradients in the cell and absorption by water itself, the thermal lensing signal strength for aqueous benzoic acid in supercritical water was found to be linear with concentration in the sub-millimolar range. Although thermal lensing experiments in aqueous media are notoriously insensitive, the sharp density gradient near the critical point considerably improves the signal intensity. In this study a short-pulse pump 266 nm YAG laser and continuous low-power probe Ar ion beam were both focused into a supercritical water cell, giving a lensing signal whose strength could be maximized by changing the overlap of the two beams.

Differential thermal lensing in extended samples.

We present a new scheme for a pump-probe mode-mismatched thermal lensing experiment aimed at extracting the value of low-absorbing components in the presence of a high-absorbing background. The method allows for the use of large path-length samples. Changes in the background absorption of a column of ethanol of 20 cm of 1% and a pollutant concentration detection limit of 0.04 ppm are obtained when using a pump light power of 34 mW.

Studies of the triplet state of the proton-transfer tautomer in salicylaldehydes

The spectroscopy and dynamics of the low-lying triplet state of the proton-transfer tautomer in salicylaldehydes have been studied via internal heavy-atom effects coupled with a sensitive near-IR detecting system. For 3,5-diiodosalicylaldehyde a weak proton-transfer keto-tautomer phosphorescence was resolved with a maximum at 710 nm ( τ p ∼1.8 μs , Φ obs∼5.23×10 −4) in a 77 K methylcyclohexane glass. The results, in combination with the time-resolved thermal lensing experiment, further deduced the triplet-state population yield and radiative decay rate to be 0.20 (298 K) and 3.12×10 2 s −1 , respectively. Consequently, the energetics and dynamics of the triplet states during a proton transfer cycle are discussed in detail.

Development of an organic dye solution for laser cooling by anti-Stokes fluorescence

ABSTRACTSeveral independent groups have observed optical cooling by means of anti-Stokes luminescence in condensed media. The most promising materials are grouped into two categories: ion-doped glasses and organic dye solutions. It is this latter group that we focus our efforts on. Recent studies by our group show that irradiating a solution of rhodamine 101 in the long wavelength wing of the absorption spectrum results in the observation of optical cooling. To improve upon the initial observation of a few degree drop in temperature requires a better understanding of the conditions and phenomena leading to anti-Stokes luminescence in dye solutions. We develop a thermal lensing experiment to obtain fluorescence quantum yields of various dye solutions. The importance of concentration, choice of solvent, deuteration, and acidification are discussed.

Density-induced heterodyne-amplified rotational dynamics: a new technique for studying orientational relaxation

A new time-domain method, density-induced heterodyne-amplified rotational dynamics (DIHARD), for studying the orientational relaxation of pure or mixed viscous liquids is described. Extremely weak absorption of an excitation pulse produces a density perturbation like that in a thermal lensing experiment. Through translational–rotational coupling, an orientational anisotropy is produced, which is heterodyne detected with a weak probe field. The DIHARD signal appears with the time for the velocity of sound to cross the excitation spot size and decays with the reorientational dynamics of the sample. DIHARD can provide data in situations where laser-excited optical Kerr experiments are difficult to perform.

Ground-State Reverse Double Proton Transfer of 7-Azaindole

Dynamics of the ground-state reverse proton transfer in 7-azaindole (7AI(N)) have been investigated by two-step laser-induced fluorescence (TSLIF) in various nonpolar solvents. Comprehensive analyses reveal a previously unrecognized finite rise kinetics for the long-lived transient species. Furthermore, the time-dependent spectral evolution indicates that the TSLIF spectrum obtained at the rise component is different from that of the decay component, while both spectra are red shifted relative to the prompt tautomer emission. The results lead us to propose that the transient species originates from the monomer of the 7AI proton-transfer tautomer (7AI(T)) produced by a minor dissociation channel (∼4%) of the excited 7AI(T) dimer, which subsequently undergoes a slow reverse proton transfer via the formation of a 7AI(T)/7AI(N) hydrogen-bonded complex. This proposed mechanism rationalizes the recent thermal lensing experiment which concluded that the 7AI(T) dimer is only 0.97 kcal/mol higher in energy than the 7AI(N) dimer,35 while theoretical approaches,38,39 in contrast, predict an energy difference of >20 kcal/mol.

Laser and spectral properties of Cr, Tm, Ho:YAG at 2.1 mu m

The authors study the loss process in a uniformly pumped amplifier experiment. The impact of the loss mechanism on laser performance is discussed and its dependence on excited state density is considered. An extensive set of measurements was performed on flashlamp pumped YAG crystals doped with chromium, thulium, and holmium. Near-room-temperature laser operation on the 2.1 mu m holmium transitions was characterized using a frequency selective resonator. Small signal gain and stored energy lifetimes were analyzed to determine the strength and dependence of loss mechanisms. The analysis of the thermal lensing experiments demonstrated a high degree of excess heat generation in this material. >

Thermal lensing spectrophotometry of uranium (VI) with pulsed laser excitation

A double-beam thermal lensing experiment based upon a pulsed dye laser as the excitation beam and a He-Ne laser as a probe beam is described. The technique has been used to detect the small absorptions given by uranyl ions in aqueous solution. The system is capable of detecting absorption coefficients as low as 5 × 10–7 cm–1. Moreover, by changing several dyes, an absorption spectrum can be obtained at concentration levels (ca. 4 × 10–6M) far below (ca. 103×) those obtained by conventional absorption spectrophotometry, thereby allowing the direct study of the chemical equilibria involved. The sensitivity of the present apparatus is limited by short-term fluctuations of the probe laser.

Vibrational energy flow in CD3H and CD3H–polyatomic mixtures

Laser-induced fluorescence studies showed that the higher CD3H vibrational fundamentals (ν2 at 2142 cm−1 and ν4 at 2263 cm−1) are populated more rapidly than the intermediate fundamental (ν5 at 1291 cm−1) when ν6 at 1037 cm−1 is excited by a Q-switched CO2 laser. The experimentally observed rate for the (ν2,ν4) fluorescence at 4.5 μ is 300±50 msec−1 torr−1. Possible activation pathways are identified using the results of thermal lensing experiments and studies of fluoresence intensity as a function of laser power. Interspecies energy transfer rates from CD3H to SF6, SiF4, and CF4 were determined to be 154±7, 146±9, and 33±1 msec−1 torr−1, respectively.