The dynamics of polarized fluorescence in reduced nicotinamide adenine dinucleotide (NADH) at 460 nm under two-photon excitation at 720 nm by femtosecond laser pulses in water−methanol solutions has been studied experimentally and theoretically as a function of methanol concentration. A number of fluorescence parameters have been determined from experiment by means of the global fit procedure and then compared with the results reported by other authors. A comprehensive analysis of experimental errors was made. Ab initio calculations of the structure of NADH in water and methanol and of β-nicotinamide mononucleotide (NMNH) in vacuum have been carried out for clarifying the role of decay time heterogeneity. The main results obtained are as follows. An explanation of the heterogeneity in the measured fluorescence decay times in NADH has been suggested based on the influence of the internal molecular electric field in the nicotinamide ring on nonradiative decay rates. We suggest that different charge distributions in the cis and trans configurations result in different internal electrostatic field distributions that lead to the decay time heterogeneity. A slight but noticeable rise of the fluorescence decay times τ 1 and τ 2 with methanol concentration was observed and treated as a minor effect of a nonradiative relaxation slowing due to the decrease in solution polarity. Relative concentrations of the folded and unfolded NADH conformations in solutions have been determined using a new method of analysis of the rotational diffusion time τ r as a function of methanol concentration on the basis of the Stokes−Einstein−Debye equation. The analysis of the fluorescence anisotropy parameters obtained under linearly and circularly polarized excitation and the parameter Ω has been carried out and resulted in the determination of the two-photon excitation tensor components and suggested the existence of two excitation channels with comparable intensities. These were the longitudinal excitation channel dominated by the diagonal tensor component S zz and the mixed excitation channel dominated by the off-diagonal tensor components |S xz 2 + S yz 2 | 1/2 .

Ab initio potential energy curves, transition dipole moments, and spin-orbit coupling matrix elements are computed for HBr. These are then used, within the framework of time-dependent quantum-mechanical wave-packet calculations, to study the photodissociation dynamics of the molecule. Total and partial integral cross sections, the branching fraction for the formation of excited-state bromine atoms Br(2P(1/2)), and the lowest order anisotropy parameters, beta, for both ground and excited-state bromine are calculated as a function of photolysis energy and compared to experimental and theoretical data determined previously. Higher order anisotropy parameters are computed for the first time for HBr and compared to recent experimental measurements. A new expression for the Re[a1(3) (parallel, perpendicular)] parameter describing coherent parallel and perpendicular production of ground-state bromine in terms of the dynamical functions is given. Although good agreement is obtained between the theoretical predictions and the experimental measurements, the discrepancies are analyzed to establish how improvements might be achieved. Insight is obtained into the nonadiabatic dynamics by comparing the results of diabatic and fully adiabatic calculations.

We present the experimental and theoretical study of the two-photon excited polarized fluorescence of p-terphenyl dissolved in cyclohexane/paraffin. The fluorescence was produced within a two-color two-photon (2C2P) excitation scheme utilizing simultaneous absorption of two femtosecond laser pulses at 400 nm and at 800 nm with the total excitation energy of 4.649 eV. The fluorescence was detected by a time correlated single photon counting (TCSPC) system with two detectors. Using different combinations of the absorbed photon polarizations we extracted seven time-dependent molecular parameters from experiment that contain all information on the dynamics of the three-photon process under study. The analysis of the obtained molecular parameter values was based on the ab initio calculations of the vertical excitation energies and transition matrix elements in p-terphenyl and allowed for determination of the whole structure of the two-photon absorption tensor, fluorescence lifetime, and the rotational correlation time. The obtained results imply that the fluorescence in the conditions of our experiment was governed mostly by the d(z) component of the fluorescence transition dipole moment that is parallel to the molecular long axis Z. The tensor was found to be symmetric. The two-photon excitation in p-terphenyl occurs simultaneously via two channels, one of them resulting in the population of the totally symmetric excited state and the other in the population of the nontotally symmetric excited state. Moreover, the energetically allowed pure electron transitions are dipole forbidden and become allowed by vibronic coupling.

We present a general method for determination of the photofragment K=4 state multipoles in an ion imaging experiment. These multipoles are important for determining the full density matrix for any photofragment with j(a)> or =2. They are expressed in terms of laboratory frame anisotropy parameters that have distinct physical origins and possess characteristic angular distributions. The explicit expression for the (2+1) resonant multiphoton ionization absorption signal for the case of arbitrarily polarized probe light is derived and a procedure for isolation of the rank-4 state multipoles from all others is shown. This treatment is applied to the case of O((1)D) produced in the 193 nm photodissociation of N2O. The results show nonzero values for all K=4 anisotropy parameters, indicating the complexity of the photodissociation dynamics in this system.