Excited State Levels Research Articles

Polarization spectroscopy of Cr 3+ ions in LiNbO 3 single crystals: effect of Mg 2+ ions

This paper discusses the polarized absorption and luminescence spectra of Cr 3+ -doped LiNbO 3 and Cr 3+ -doped MgO : LiNbO 3 single crystals. In Cr 3+ -LiNbO 3 the broad-band 4A 2, 4T 2 absorption transition is strongly σ polarized as anticipated for sites with C 3 (nearly C 3v) local symmetry. For this same material the sharp R-lines associated with the 4A 2 ↔ 2E transitions are also σ polarized, due to admixing of the almost degenarate 4T 2 and 2E excited state levels. In contrast with this behaviour, both 4A 2 ↔ 4T 2 and 4A 2 ↔ 2E transitions in Cr 3+ -MgO:LiNbO 3 are observed to be π polarized, suggesting that the local symmetry of Cr 3+ centres is perturbed by the close proximity of Mg 2+ ions.

From classical to quantum glass.

We study the effects of a transverse magnetic field on the dynamics of the randomly diluted, dipolar coupled, Ising magnet ${\mathrm{LiHo}}_{0.167}$${\mathrm{Y}}_{0.833}$${\mathrm{F}}_{4}$. The transverse field mixes the eigenfunctions of the ground-state Ising doublet with the otherwise inaccessible excited-state levels. We observe a rapid decrease in the characteristic relaxation times, large changes in the spectral form of the relaxation, and a depression of the spin-glass transition temperature with the introduction of quantum fluctuations.

Relaxation behind shock waves in ionizing neon

Experiments in ionizing neon are described in which heavy-particle and electron densities are measured and compared with the current kinetic model. The experimental results allow a determination of the atom-atom collisional excitation cross-section constant, giving a value of (8±2) × 10−20 cm2/eV for the range of conditions examined. The population of two of the excited-state levels of the atom are measured and compared with theoretical predictions. The experimental populations are found to follow the expected behaviour in the quasi-equilibrium region, but are several orders of magnitude higher than predicted in the nonequilibrium zone. These findings suggest that the depopulation of excited states through ionizing collisions occurs more slowly than expected. Absorption linewidths and line shifts are also measured in the quasi-equilibrium region and found to compare well with theory. A small precurson population is also observed and population measurements in this region are compared with other experimental results in argon and krypton.

Pyrolysis jet spectroscopy: laser-induced phosphorescence of thioformaldehyde and the triplet excited-state bending potential

Rotationally cold laser-induced phosphorescence spectra of thioformaldehyde have been obtained by using the pyrolysis jet technique. Rotational constants, including the major spin constants, have been obtained for three excited-state rotational levels. The jet data have been combined with previously available data from room-temperature spectra of H{sub 2}CS and D{sub 2}CS and fitted to a semirigid invertor Hamiltonian. The results show that it is not necessary to invoke an excited-state inversional barrier in the potential function in order to accurately fit the data. Thioformaldehyde adopts a planar excited triplet state equilibrium geometry, as was also concluded in the authors previous studies of the excited singlet state.

Lifetime‐Measurements of Selectively Excited Molecular Levels

AbstractA synchroneously pumped mode‐locked cavity‐dumped dye laser, which delivers nearly Fourier‐limited output pulses with 20 W peak power, 500 ps pulsewidth and 1.5 GHz spectral bandwidth is used to populate selected rovibronic levels in excited electronic states of alkali molecules. The laser induced fluorescence is filtered through a monochromator and the lifetimes of individual rovibronic levels are determined with the single photon delayed coincidence technique. The influence of different perturbations on the lifetimes is investigated and discussed.

Dynamics of Quantum Selected Levels of Electronic Excited States of Small Molecules

Dynamics studies of the electronic excited states of some small molecules have been carried out. Collision free lifetimes of different rotational and vibrational levels of the A and B states of 130Te2 and 80Se2 have been obtained. Predissociation studies of the B states of the Br2 and Cl2 molecules have been carried out. Results are discussed in terms of perturbation or predissociation effects.

Gas-phase molecular energy transfer studies with time-resolved spectroscopy

The emission spectrum of molecular nitrogen excited in pulsed electric discharges has been examined by means of time-resolved spectroscopy. Thereby we obtain a spectrum characteristic of a particular moment in time or, alternatively, a record of the emitted light as a function of time at a particular wavelength. The experiments consist of establishing appropriate atmospheres of flowing gas and imposing on them electrical excitation pulses of various voltages, widths, and repetition rates. Meanwhile, the molecular emission is monitored throughout the period of excitation, and beyond, in order to track the distribution of excited-state level populations as they evolve in time and under the influence of collisions. The time resolution of the methods employed here has been chosento provide for the evaluation of molecular population distributions on a collision-by-collision basis, which for present purposes is of the order of 1 μsec. Examining both changing spectra and the time signatures of individual emitting species, we attempt to interpret the processes underlying shifts in the molecular populations in a pulsed electric discharge.

Optical-pumping effects on Raman-heterodyne-detected multipulse rf nuclear-spin-echo decay.

Raman-heterodyne-detected multipulse rf nuclear-spin-echo decay of ${\mathrm{Pr}}^{3+}$ in ${\mathrm{YAlO}}_{3}$ is measured with continuous-detection light throughout the spin-locking pulse train and with the light on during a short period centered at the echo time only for every sixth echo. The echo decay shows three components: an initial fast decay (\ensuremath{\simeq}400 \ensuremath{\mu}s), an intermediate decay (5.4\ifmmode\pm\else\textpm\fi{}0.6 ms), and a slower decay of 22.5\ifmmode\pm\else\textpm\fi{}1.7 ms for gated echoes. For cw light, the decay is described by the same initial fast decay, but a single decay at 7.6\ifmmode\pm\else\textpm\fi{}0.6 ms is observed at longer times. The difference in the observed echo decay is attributed to optical pumping effects. A rate-equation model is constructed, consisting of three ground-state levels and three excited-state levels. It includes radiative decay, spin-lattice relaxation, the optical pumping and rf pulses, and rf coherence loss due to excited-state occupancy. The model shows three decay times, fast (\ensuremath{\simeq}400 \ensuremath{\mu}s), intermediate (6.08\ifmmode\pm\else\textpm\fi{}0.07 ms), and slow (21.0\ifmmode\pm\else\textpm\fi{}0.1 ms) decay for gated echoes. The last two rates are affected by the strength and duration of the optical field. For continuous light, the calculated echo is seen to decay at \ensuremath{\simeq}400 \ensuremath{\mu}s, 2.04\ifmmode\pm\else\textpm\fi{}0.07 ms, and 3.43\ifmmode\pm\else\textpm\fi{}0.02 ms and then increases at a much slower rate (100 ms). This is only an apparent increase, because significant signal cancellation occurs in the sum of the six ion groups contributing to the signal. If a reduced light level and faster spin-lattice relaxation rates are assumed, the calculated echo-decay times for cw light are 2.44\ifmmode\pm\else\textpm\fi{}0.03 and 7.37\ifmmode\pm\else\textpm\fi{}0.04 ms. This corresponds to a homogeneous linewidth increase of 2.5 times and a spin-lattice relaxation rate increase of 2.5 times equivalent to crystal heating by the cw detecting light of 0.35 K over that of the gated light.

Time-resolved spectroscopy of BaFBr:Eu 2+

The time and wavelength dependence of the Eu 2+ 5d-4f emission in BaFBr was measured over the extended temperature range 10–506 K. Excitation was provided by either a nitrogen laser (337 nm)or the third harmonic output of a Nd +-YAG laser (355 nm). At 10 K, the measured emission lifetime is (500±5) ns; at 506 K, the lifetime increases to (968±5) ns. In our kinetic analysis, three parameters are required to fit the temperature dependence of the emission lifetime. We determine the low-temperature lifetime limit, the difference in energy for two relatively close-spaced excited state levels and the degeneracy ratio g -1 , for the two levels. Our best fit to the data yields (508±7) ns, (-476±8) cm -1, and (3.5±0.1), respectively, for these parameters. Corrected cw emission spectra show that the lineshape is asymmetric at low temperatures; this asymmetry is greatly reduced as the temperature is increased. The spectral dependence of the emission lifetime, however, shows no such asymmetry. There is no variation of the emission lifetime with wavelength, throughout the band, regardless of temperature, as expected for direct substitution of Eu 2+ for the Ba 2+ in this lattice.

Energy Levels of the Single Excited States in the Magnesium Isoelectronic Sequence

Energy Levels of the Single Excited States in the Magnesium Isoelectronic Sequence

Energy Levels of the Single Excited States in the Boron Isoelectronic Sequences

AbstractEnergy levels of the single excited 1s22s2ns (2S), 1s22s2np (2P), 1s22s2nd (2D) and 1s22s2nf (2F), n = 3‐8 states for the boron isoelectronic sequence are calculated using the simple configuration Hartree‐Fock method. Good agreement is obtained between our results and previous experimental and theoretical data.

Optical–optical double resonance (stimulated emission pumping) spectroscopy of HCF

Optical–optical double resonance or stimulated emission pumping spectroscopy has been applied to the HCF molecule to obtain precise molecular constants in excited vibrational states associated with the ground electronic state. A cw dye laser excites HCF molecules to a single rotational level of the Ã1 A″(000) state, and transitions induced by a second cw dye laser downward to rotational levels in excited vibrational states of the ground electronic state are observed as dips of the fluorescence from the Ã1 A″(000) state. The signals are observed at sub-Doppler resolution (about 0.002 cm−1 FWHM), but their wave numbers are determined only to the precision 0.003 cm−1 of the dye laser wave number measurement. Molecular constants in the (010) and (020) states are determined precisely by a conventional least-squares analysis of the observed signals. The (100) state is found to interact with the (011) state through Coriolis coupling, and the spectral data obtained for the two states are analyzed simultaneously. The ν1 band origin thus determined for the first time is unusually low, i.e., 2643.0393 (26) cm−1 with one standard deviation in parentheses.

Precision measurement of the Stark shift of the cesium D lines.

The dc Stark shift of the cesium D lines is observed in the low-field limit where the induced shift is small compared with the various hyperfine splittings. On the D2 line, modification by the electric field of the dipole transition rates to the various excited-state hyperfine levels must be taken into account in order to interpret our results. From our observations the following atomic polarizabilities in units of ${10}^{\mathrm{\ensuremath{-}}24}$ ${\mathrm{cm}}^{3}$ are deduced: ${\mathrm{\ensuremath{\alpha}}}_{0}$(${6\mathrm{P}}_{1/2}$)-${\mathrm{\ensuremath{\alpha}}}_{0}$(${6\mathrm{S}}_{1/2}$)=143.8\ifmmode\pm\else\textpm\fi{}1.4, ${\mathrm{\ensuremath{\alpha}}}_{0}$(${6\mathrm{p}}_{3/2}$)-${\mathrm{\ensuremath{\alpha}}}_{0}$(${6\mathrm{S}}_{1/2}$)=187.3\ifmmode\pm\else\textpm\fi{}1.9, and ${\mathrm{\ensuremath{\alpha}}}_{2}$(${6\mathrm{P}}_{3/2}$)=-38.6\ifmmode\pm\else\textpm\fi{}1.2.

Direct determination of the adiabatic ionization potential of NO2 by multiresonant optical absorption

The adiabatic ionization threshold of NO2 is observed by direct optical excitation in a three-color, triply resonant, three-photon transition from vibrationless levels of the bent ground state through the vibrational ground state of the linear Rydberg state to the ground state of the ion. The overall energy for this process for rotationless molecules is estimated to be 77320 ± 20 cm−1. The bent-to-linear two-photon transition to the Rydberg state is facilitated by a first-photon resonance with levels of NO2's mixed low-lying excited states. At higher energies, following intermediate selection of vibrationally excited 3pσ Rydberg states, transitions to series of high Rydberg states are observed uniformly converging to respective vertical ionization potentials. These transitions are marked by sharp bands below and in some cases above the adiabatic ionization threshold. Intensities suggest efficient autoionization.

Laser photofragment spectroscopy of near-threshold resonances in SiH +

Near-threshold resonances in the photodissociation cross section of SiH + have been recorded by the detection of photofragment Si + ions. The photodissociation spectrum is found to be dominated by Feshbach resonances. The first evidence for multichannel resonances in the photodissociation spectrum of a diatomic molecule is presented. Laser photodissociation spectra between 15600 cm -1 and 18750 cm -1 were recorded at a resolution of 0.0012 cm -1 by coaxial laser irradiation of a fast ion beam of SiH + . Over 70 transitions were observed, the majority of which involve excited-state levels (resonances) that lie between the Si + (2P|) + H ( 2 S) and Si+(2Pi) + H ( 2 S) dissociation limits, which are separated by 287 cm -1 . The assignments were made by combining experimental information with predictions of the vibrational and rotational energy levels obtained by numerical solution of the radial Schrodinger equation and calculations of the rotational line intensities. The experimental data were obtained by measurement of the transition frequencies, line intensities, line widths and hyperfine splittings and through examination of the effect of laser power on the linewidth. The kinetic energy released on dissociation and the photofragment angular distribution were also determined.

A chemical process producing a continuous laser amplifier in the visible region

In a study of the reaction dynamics of the Na 3-X (Cl, Br, I) metatheses, Na 2 * is formed in several excited electronic states, the fluorescence from these states resembling in some respects that characteristic of optically pumped alkali dimer lasers. Optical gain through stimulated emission demonstrates population inversion involving several excited state levels of Na 2 and hence an amplifying medium for a visible chemical laser. A maximum laser gain of 1% of the input laser power has been observed at 5270 Å, corresponding to a gain coefficient of ≈ 2 × 10 −3 cm −1. An explanation is provided for this efficient inversion process and comparisons are made with optically pumped laser systems.

Experimental determination of Na–noble-gas velocity-changing and fine-structure-changing collision kernels

We report on an experimental investigation of the collision kernels governing the effects of velocity-changing collisions, fine-structure-changing collisions, and their interplay, for Na--noble-gas pairs. The velocity distributions in the two Na excited-state fine-structure levels and in the Na ground-state hyperfine levels have been measured in the presence of a pump laser which produces velocity-selective excitation. For interpretation of these experiments we use a rate-equation model developed in the context of light-induced drift: the velocity-changing collisions are accounted for by a composite Keilson-Storer kernel describing large-angle and small-angle collisions and the fine-structure-changing collisions are treated in the sudden limit. The model is valid for arbitrarily high pump laser intensities and uses only a single adjustable parameter characterizing the small-angle scattering. It is found that the theory describes the experimental velocity distributions in all four Na levels over a wide range of experimental parameters for each Na--noble-gas pair. The relevance of the collision kernels deduced in the present work is discussed in the context of light-induced drift.

Quantum jumps via spontaneous Raman scattering.

A single laser, which is used to induce and detect spontaneous Raman transitions, can be used to observe quantum jumps in a single atom. The population dynamics of a particular system, consisting of two $^{2}\mathrm{S}_{1/2}$ ground-state levels and four $^{2}\mathrm{P}_{3/2}$ excited-state levels split by a magnetic field, is analyzed for a laser tuned near a particular transition. We find that the statistics of the fluorescence emitted by this system are described by the same formalism developed for the three-level V configuration irradiated by two light sources. Over a wide range of observation times, the fluorescence intensity will be two valued, either off or on, as has been verified for the V configuration. Some surprising and elegant features of this new system are described.

Observation of forbidden transitions of ammonium ion (NH+4) ν3 band and determination of ground state rotational constants. Observation of ν3 band allowed transitions of ND+4

The high resolution infrared spectrum of forbidden transitions of the ν3 band of ammonium ion, NH+4, has been recorded using a difference frequency spectrometer. These are the first forbidden NH+4 lines to be observed. For 5≤R≤11, 24 forbidden transitions have been assigned. From combination difference relations involving the same excited state level, 13 independent energy differences between ground state levels with 4≤J≤11 have been determined with uncertainties of ≲0.004 cm−1. From this data, the following values (in cm−1) for ground state constants of NH+4 have been determined (±3σ):B=5.929 32 ±0.000 20, D=(−1.282±0.020)×10−4, and Dt=(4.80±0.35)×10−6. The value of r0 is calculated to be 1.028 73±0.000 02 Å, while re is estimated to be 1.0208±0.0020 Å. More than 100 transitions of the ν3 band of ND+4 were also observed (J≲10) by the same spectrometer. Nearly all of these have been assigned and fit together using a linear model. A determination of B0 for ND+4, while unsuccessful at this time, will allow a more accurate determination of re.

Two-photon, carbon-13 and two-dimensional proton NMR spectroscopic studies of retinyl Schiff bases, protonated Schiff bases and Schiff base salts: evidence for a protonation induced .pi..pi.* excited state level ordering reversal

The ..pi pi..* excited singlet state manifolds of the visual chromophores, all-trans-retinylpyrrolidiniminium perchlorate (ATRSBS) and all-trans-N-retinylidene-n-butylimine:HCl (ATRPSB) are studied by using one-photon and two-photon laser spectroscopy. The goal is a better understanding of how protonation and counterion location affect level ordering in retinyl Schiff bases. Ambient temperature two-photon thermal lensing spectra indicate that ATRSBS has a lowest lying /sup 1/A/sub g/*/sup -/-like state as was observed previously for all-trans-retinal and the Schiff base of all-trans-retinal. In contrast, two-photon spectra of ATRPSB indicate that the protonated Schiff base has a lowest lying /sup 1/B/sub u/*/sup +/-like state. The origin of this level ordering reversal is analyzed by using molecular orbital theory as well as /sup 13/C and two-dimensional /sup 1/H NMR. They conclude that the relative level ordering of the low-lying covalent and ionic ..pi pi..* excited states of protonated Schiff bases and Schiff base salts is highly sensitive to counterion location (diffuseness). INDO-PSDCI molecular orbital theory is shown to be a reliable theoretical method of predicting the effect of counterion location on the one-photon and two-photon properties of retinyl protonated Schiff bases and Schiff base salts. This study provides further experimental support for the conclusions of a previous two-photon investigationmore » of the rhodopsin binding site which demonstrated that the protein bound 11-cis-retinyl chromophore is protonated and occupies a neutral binding site.« less