Double Resonance Transitions Research Articles

An optical–optical double resonance spectroscopy of Br2: Observation of the 1g(3P2) ion-pair state

We report the first observation of the 1g(3P2) ion-pair state of Br2 by the optical–optical double resonance technique. This state is accessible from the ground state stepwisely in the pump–probe excitation scheme, 1g(3P2) –A 3Π(1u) –X 1Σ+g. The double resonance transitions were interpreted on the basis of the optical selection rules. Emission spectra of the 1g(3P2) –A 3Π(1u) system were interpreted to establish the absolute v′ numbering of the ion-pair state from the Franck–Condon factor calculations. Observations were made on the v′=0 through v′=10 levels of the 79Br2 isotope species. The molecular parameters obtained are Te =49 390.364(16), ωe =151.704(10), ωexe =0.4169(24), ωeye =4.7(16)×10−4, Be =0.042 1840(73), αe =1.5209(61)×10−4, De =1.32(10)×10−8, and q=4.91(19)×10−5 (all in cm−1 with 3σ in parentheses).

Optical–optical double resonance and double resonance multiphoton ionization in 7Li2 produced by single cw dye laser excitation

A comparison of fluorescence detection of optical–optical double resonance transitions with ionization detection of double resonance multiphoton ionization signals was made for Li2 vapor. The spectra in both cases were excited by a single broadband (30 GHz) cw dye laser and exhibited identical features. These arise from ‘‘accidental’’ two-photon double resonance transitions via real intermediate states and consist of hundreds of distinct transitions in the region 565–645 nm studied in this work. Analysis and assignment reveal that multiphoton excitations within the singlet manifold of electronic states of Li2 together with intermediate state rotational relaxation can account for the spectra. A table of vibrational and rotational assignments of the more intense features is available which includes a comparison between calculated and observed intensities for the transitions.

First observation of the f(0+g) ion-pair state of Cl2 by optical–optical double resonance

The two-photon excitation spectrum of Cl2 using two tunable dye lasers has been studied in the 7 eV energy range. The f(0+g) ion-pair state correlating with Cl−(1S)+Cl+(3P0) at the dissociation limit appeared in the spectrum resulting from an optical–optical double resonance transition through the B 3Π(0+u) state. The absolute vibrational numbering of the f(0+g) state was established by direct observation of the f(0+g)–B 3Π(0+u) fluorescence. The molecular constants for the Cl2 f(0+g) state are derived (in cm−1 and 3σ in parentheses): Te=59 356.124(41), ωe=256.981(30), ωexe=1.2032(83), ωeye =0.003 69(47), Be=0.115 03(10), αe=7.58(17)×10−4, and D=1.44(68)×10−7.

Resonant multiphoton ionization of NO via the A 2Σ+ state: Photoelectron spectra and angular distributions

Photoelectron spectra resulting from the two-photon resonant, four-photon ionization of NO via the A 2Σ+ state are reported for several rotational levels in the A, v=0 manifold. The observed final ionic state vibrational distributions are found to be consistent with and confirm spectroscopic studies which have assigned intense features in the four-photon A state MPI spectrum with accidental resonances in the energy range of the third photon. The upper states in the strong accidental double resonance transitions are assigned to Rydberg vibronic levels which are strongly perturbed by nearby valence states. Photoelectron angular distributions were also measured for two vibronic transitions and these are interpreted on the basis of a simple, one-electron description of the ionization step.

The optical–optical double resonance spectrum of 7Li2 excited by a single tunable cw laser

The two-photon excitation spectrum of 7Li2 produced by a single tunable cw dye laser operating between 570 and 650 nm has been observed using ultraviolet fluorescence detection. The spectrum arises from optical–optical double resonance transitions from the X 1Σg+ ground electronic state to the F 1Σg+ and G 1Πg excited states via the A 1Σu+ intermediate state, in which vibrational and rotational relaxation also take place. Using the known molecular constants for these states, a theoretically predicted spectrum was found from which the prominent lines in the experimental spectrum could be assigned. The multiphoton ionization spectrum of Li2, previously observed and interpreted in terms of bound–free–bound triplet absorption bands, can be explained as OODR transitions among the above states followed by photoionization.

The optically detected magnetic resonance spectrum of BO 2 in the gas phase

Using 5145 Å radiation from an argon ion laser, microwave optical double resonance transitions have been observed among Zeeman-tuned rovibronic sublevels of the X 2Π state of BO 2. Analysis of the strongest of the observed transitions has permitted a more accurate measurement of the Renner parameter than has previously been possible for the ground state of this molecule.

Measurement of thev2infra-red band of silane, and a simultaneous analysis of thev2andv4bands

The infra-red spectrum of silane has been recorded with a resolution of about 0·06 cm-1 between 950 and 1076 cm-1. This region contains most of the stronger v 2 transitions and the upper part of the v 4 R branch. The v 2 spectrum has been assigned up to J′ = 20, and the assignment given previously for v 4 has also been extended to this value of J. The data for both bands have been analysed by a simultaneous diagonalization of the effective hamiltonians for the v 2=1 and v 4=1 states coupled by the Bζ2,4 Coriolis interaction term. This model is successful in reproducing the observations; 645 assigned transitions are fitted with a standard deviation of 0·017 cm-1, and the calculated intensities are in good agreement with experiment. The assignment of the laser-RF double-resonance transitions observed by Kreiner and Oka is discussed.

Collision-induced optical double resonance

In a previous paper, we demonstrated that a set of new resonances can accompany the conventional infrared Lamb-dip or double-resonance transitions in C${\mathrm{H}}_{3}$F gas. These satellite lines have a different origin from the primary resonances even though they are similar in intensity and linewidth. The traditional double resonance, for example, requires that a molecule interact simultaneously with two radiation fields, causing a transition from an initial to a final state through an intermediate level. Here, the double-resonance concept is extended to the situation of two coherently driven optical transitions that do not share a common level but are coupled by molecular collisions that tip the angular momentum vector while preserving the molecular velocity and rotational energy. Thus, velocity-selective population changes are communicated from one transition to another through collisions. Collision-induced double resonance is observed with Stark tuning as a series of sharp lines, free of Doppler broadening and can be explained in the same order of perturbation theory as the ordinary double-resonance experiment. Each satellite corresponds to a specific level structure involving one or more collision-induced transitions among the space-quantized $M$ states. Virtually all characteristics of these satellite resonances, either in Lamb-dip or double-resonance experiments, are in agreement with the theory presented. While purely optical coupling mechanisms can yield double-resonance behavior also, such effects are estimated to be too weak. For the symmetric top molecule C${\mathrm{H}}_{3}$F, the dipole-dipole interaction of collision pairs dominates and induces the observed reorienting transitions. The corresponding cross section for low-angular-momentum states ($J,K=4,3 or 5,3$) is found to be \ensuremath{\sim} 100 ${\mathrm{A}}^{2}$ whereas for high-angular-momentum states ($J,K=12,2$), the cross section is calculated to be about 100 times smaller, and satellite resonances are not even detected. A treatment is presented also for the case where the collisionally coupled $M$ states require a sequential transfer of population over intermediate levels. The problem of velocity smearing is discussed in this context where the energy exchange in a collision is small and also for more energetic collisions that may involve rotational quantum jumps.

Dispersion of nonlinear optical susceptibility in GaAs and GaSb

Dispersion measurements of the nonlinear optical susceptibility (NLOS) of GaAs and GaSb crystals are reported. The intensity of the second-harmonic beam generated in the III-V crystal was measured as a function of the frequenvy (1.2-1.8 eV) of the radiation source. This measurement was simultaneously calibrated with respect to the second-harmonic beam generated in a reference quartz crystal. The NLOS modulus was determined from the ratio of the signals in the III-V crystal and the quartz. The dispersion measurements were carried out on mechanically polished crystals. Test measurements on etched crystals, at a few energy values, showed a change in the magnitude of the NLOS, although no significant influence on the spectral shape was found. The structure of the spectra shows good correspondence between the position of the measured peaks and the position of those peaks found in the single-photon (linear) measurements. Previous measurements in GaAs show no such correspondence. Some evidence was found for the possible existence of a two-photon double-resonance transition in GaSb. The Miller-cofficient spectra were calculated. For GaAs the spectrum was found to be approximately constant. For GaSb no pronounced structures were found.

Laser optical double resonance and efficient infrared quantum counter upconversion in LaCl3 : Pr3+ and LaF3 : Pr3+

In this paper, we report the first laser-excited optical double-resonance experiments in rare-earth-doped crystals. Infrared upconversion in LaCl3 : Pr3+ and LaF3 : Pr3+ has been achieved by means of a tunable cw dye laser pump. The quantum efficiency of the upconversion process at 2.03 μ in LaCl3 : Pr3+ is 1.2×10−3, which is an improvement of 4×105 over the highest efficiency found previously with incoherent sources. It is shown that an efficiency of 1 can be easily obtained with higher fluxes and improved crystal quality. Direct excitation of the output fluorescence from the 3P0 level is observed in both LaCl3 : Pr3+ and LaF3 : Pr3+ when the dye laser frequency is resonant with the states of the lower-lying 1D2 manifold in the absence of the infrared input. Detailed assignments of the observed double-resonance transitions have been made from wavelength and polarization measurements.

Overhauser Studies in Two-Spin Systems

Expressions have been derived for fractional changes in intensity of double resonance transitions for weak irradiation of two spin systems for random field and dipole–dipole interactions. The relative fractional changes are shown to be independent of the strength and small offsets of the irradiation and the strength of the observing field. The fractional changes are found to be quite sensitive to the relaxation mechanisms, thereby allowing a simple interpretation of the observed fractional changes in terms of the relaxation mechanism operative in the spin system. Overhauser experiments have been performed on the AX spin system of 2,2-dichloroacetaldehyde, studied in different sample conditions and the AB spin system of 1-bromo-1-chloroethylene. All the degassed samples have been shown to have contributions from both random field and dipole–dipole interactions, the magnitudes of which have been obtained in each case. An analysis of sources of errors in these experiments indicates that even for strengths of irradiation much smaller than the observed linewidths in the spectra, splittings of the connected transitions are important for quantitative estimates of the intensities. The diagonal elements of η, the change in density matrix due to the observing field, are shown to be negligible in the present experiments.