Electronic Fluorescence Spectra Research Articles

Impact of free electron degeneracy on collisional rates in plasmas

Degenerate plasmas, in which quantum effects dictate the behavior of free electrons, are ubiquitous on earth and throughout space. Transitions between bound and free electron states determine basic plasma properties, yet the effects of degeneracy on these transitions have only been theorized. Here, we use an x-ray free electron laser to create and characterize a degenerate plasma. We observe a core electron fluorescence spectrum that cannot be reproduced by models that ignore free electron degeneracy.We show that degeneracy acts to restrict the available electron energy states, thereby slowing the rate of transitions to and from the continuum. We couple degeneracy and bound electron dynamics in an existing collisional-radiative code, which agrees well with observations. The impact of the shape of the cross section, and hence the magnitude of the correction due to degeneracy, is also discussed. This study shows that degeneracy in plasmas can significantly influence experimental observables such as the emission spectra, and that these effects can be included parametrically in well-established atomic physics codes. This work narrows the gap in understanding between the condensed-matter and plasma phases, which coexist in myriad scenarios.

Temperature dependence of the hydrated electron's excited-state relaxation. II. Elucidating the relaxation mechanism through ultrafast transient absorption and stimulated emission spectroscopy.

The structure of the hydrated electron, particularly whether it exists primarily within a cavity or encompasses interior water molecules, has been the subject of much recent debate. In Paper I [C.-C. Zho et al., J. Chem. Phys. 147, 074503 (2017)], we found that mixed quantum/classical simulations with cavity and non-cavity pseudopotentials gave different predictions for the temperature dependence of the rate of the photoexcited hydrated electron's relaxation back to the ground state. In this paper, we measure the ultrafast transient absorption spectroscopy of the photoexcited hydrated electron as a function of temperature to confront the predictions of our simulations. The ultrafast spectroscopy clearly shows faster relaxation dynamics at higher temperatures. In particular, the transient absorption data show a clear excess bleach beyond that of the equilibrium hydrated electron's ground-state absorption that can only be explained by stimulated emission. This stimulated emission component, which is consistent with the experimentally known fluorescence spectrum of the hydrated electron, decreases in both amplitude and lifetime as the temperature is increased. We use a kinetic model to globally fit the temperature-dependent transient absorption data at multiple temperatures ranging from 0 to 45 °C. We find the room-temperature lifetime of the excited-state hydrated electron to be 137±40 fs, in close agreement with recent time-resolved photoelectron spectroscopy (TRPES) experiments and in strong support of the "non-adiabatic" picture of the hydrated electron's excited-state relaxation. Moreover, we find that the excited-state lifetime is strongly temperature dependent, changing by slightly more than a factor of two over the 45 °C temperature range explored. This temperature dependence of the lifetime, along with a faster rate of ground-state cooling with increasing bulk temperature, should be directly observable by future TRPES experiments. Our data also suggest that the red side of the hydrated electron's fluorescence spectrum should significantly decrease with increasing temperature. Overall, our results are not consistent with the nearly complete lack of temperature dependence predicted by traditional cavity models of the hydrated electron but instead agree qualitatively and nearly quantitatively with the temperature-dependent structural changes predicted by the non-cavity hydrated electron model.

The Diagnostics of Laser-Induced Fluorescence (LIF) Spectra of PAHs in Flame with TD-DFT: Special Focus on Five-Membered Ring.

The electronic emission characteristics of 13 gas-phase PAHs, ranging from phenlylacetylene to rubicene, were investigated to diagnose laser-induced fluorescence (LIF) spectra of PAHs in flame by DFT, TD-DFT, and premixed flame modeling methods. It was found that the maximum emission wavelengths of the PAHs with five-membered ring are located in visible region and insensitive to the number of C atoms. However, the fluorescence wavelengths of the PAHs without five-membered rings increase with the number of C atoms due to the reduced HOMO-LUMO gap. In addition, the fluorescence wavelength of the PAHs without five-membered rings with linear arrangement is longer than that of PAHs with nonlinear arrangement. According to the Franck-Condon principle, the vibrationally resolved electronic fluorescence spectra were obtained. The results show that fluorescence bandwidth of the PAHs with five-membered rings is much broader than that of the PAHs without five-membered rings. The concentration of PAHs was calculated by using the premixed flat-flame model with KM2 mechanism. On the basis of the fluorescence bandwidth and the concentration of the PAHs, the potentially fluorescence distribution of PAHs in flame was mapped. One can distinguish the specific PAHs according to the mapped fluorescence distribution of PAHs in this study. It was found that naphthalene should be responsible for the fluorescence located in the 312-340 nm region in the flame. 1-Ethynylnaphthalene is the most possible candidate to emit the fluorescence located in the 360-380 nm region. The fluorescence signals with the wavelength longer than 500 nm are likely emitted by the PAHs with five-membered rings. This study contributes to enhance the selectivity of PAHs in LIF technology, especially in the visible region.

Solvation and Spectra of a Charge Transfer Solute in Ethanol Confined within Nanoscale Silica Pores

The free energy and electronic fluorescence spectra of a model solute solvated by ethanol in a nanoscale silica pore are examined as a function of the solute position, with the aim of improving our understanding of solvation in nanoconfined environments. The results indicate that the position distribution of the solute depends on its dipole moment as well as on the surface interactions of the silica pore, i.e., hydrophilic or hydrophobic (uncharged). Further, the solute fluorescence spectrum is a function of the solute position in the hydrophilic pore, but is independent of position in the hydrophobic pore. The origins of these results are investigated, including by decomposition of the free energy as a function of solute position into the contributing interactions. The implications for time-dependent fluorescence (TDF) experiments, used commonly to probe solvation dynamics in nanoconfined solvent systems, are considered. The possible role of chromophore diffusion in TDF measurements, and chemistry in nanoconfined liquids more broadly, is given particular emphasis.

Semiempirical calculation of the absolute solvation shift of electronic and vibrational spectra of molecules in the gas-solution phase transition

Calculations of the main components of the absolute solvation shift of Δυ∑a, f, R optical absorption, emission, and Raman spectra of molecules in the gas-solution phase transition for solutions of several diatomic (HCl, DCl, H2, D2) and polyatomic (anthracene, 3-and 4-aminophthalimide) substances in some polar and nonpolar solvents are presented. The calculations are made by a new method of the semiempirical theory, which was proposed by the author earlier and makes such calculations possible for the first time. It is shown that the given theory adequately describes the photophysical effect under study and provides a correct prediction of the values of Δυ∑a, f, R, which differ for different systems by more than three orders of magnitude (from several inverse centimeters for vibrational spectra of H2 and D2 to many thousands of inverse centimeters for electronic fluorescence spectra of solutions of aminosubstituted derivatives of phthalimide).

Electronic to vibrational energy transfer and relaxation in matrices. II. Hg in mixed N 2/Kr matrices

The electronic fluorescence spectrum of Hg-doped N 2/Kr mixed matrices obtained by laser excitation of the Hg 3P 1 level is composed of the Hg “atomic” ( 3P 1— 1S 0 and 3P 0— 1S 0) fluorescence and the exciplex fluorescence due to the (Hg—N 2) * complex stable in the excited state. The temperature dependence of their intensities and lifetimes was studied in the temperature range 12–24 K. It is argued that the essential part of the 3P 0 and exciplex emission is due to two types of Hg sites with one N 2 nearest neighbor, differing probably in the orientation of the N 2 molecular axis. Strong irreversible effects due to the diffusion of N 2 molecules induced by laser irradiation are observed.

Polarization effects on rotational line strengths in collision-free polyatomic fluorescence spectra

Methods are given to calculate rotational line strengths in collision-free polyatomic electronic fluorescence spectra after single rotational levels are pumped with polarized (laser) excitation. Such fluorescence retains substantial polarization so that attempts to calculate line strengths with standard Hönl–London formulas or related procedures give significant errors. Illustrations with glyoxal S1→S0 fluorescence show good agreement between observed and calculated line strengths when polarization is accounted for. Specific formulas are given for the separate polarization components when partial depolarization of such fluorescence also occurs on account of external magnetic fields. Comparisons of field-free calculations with experimental data show that cross sections for collisional fluorescence depolarization (ΔM changes) in S1–S0 glyoxal collisions do not exceed those known for ΔJ, ΔK changes (about five times gas kinetic).

Excited states of six-membered aza-aromatic rings. Part VIII. Photochemical reactions, fluorescence, and protolytic equilibria in N-alkylated phenazinium ion–phenazyl free radical systems

Electronic fluorescence spectra of the N-methylphenazyl free radical and its protonated form are reported. The protolytic pK values for the ground and first excited states of the species have been determined and compared with data on a series of related compounds. A hitherto obscure difference in the reduction mechanism of N-methyl and N-ethyl-phenazinium ions is explained by steric inhibition of the reaction path leading to dealkylation.