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The 3dδ 1[uppercase_phi_synonym]g and 3[uppercase_phi_synonym]g Rydberg states of O2 have been observed by (2+1) resonant enhanced multiphoton ionization from the metastable a 1Δg state and the resulting spectra rotationally analyzed. In contrast to all other observed Rydberg states of O2, the 3[uppercase_phi_synonym]2, 3[uppercase_phi_synonym]3, and 3[uppercase_phi_synonym]4 states show no sign of perturbation or predissociation by valence states. The 1[uppercase_phi_synonym]3 state is apparently weakly predissociated. The energy level structure of those 3d Rydberg states which have been reliably assigned is compared with theoretical calculations. The reasons for the failure to detect Πg states in these experiments are discussed. [ABSTRACT FROM AUTHOR]

The v’=0–4 levels of the 3dπ 1Δg state and v’=0–3 levels of the 3dπ 3Δ2g state of O2 have been identified and studied by multiphoton ionization and polarization techniques. The observed levels exhibit no detectable perturbation except for rather large Λ doubling probably due to L uncoupling, and heterogeneous predissociation probably by a Πg valence state. The observation of large amounts of O+ for the higher vibrational levels suggests large non-Franck–Condon effects in the ionization process. [ABSTRACT FROM AUTHOR]

Time-delayed optical double resonance multiphoton ionization (MPI) studies involving (4pπ)K 2Π(v=2)←(3sσ)A 2Σ+(v=1) ←←X 2Π(v=0) NO show dramatic loss of the ionization signal amplitudes on a time scale that is very short relative to the lifetime of the A state. From direct comparisons of MPI and laser induced fluorescence (LIF) dip temporal profiles, it is concluded that loss of the signal amplitude results from inefficient ionization of the K state. MPI temporal profiles measured in a supersonic jet are identical with those measured in a cell indicating that collisional effects are not important. For delay times τ≥20 ns, ionization signals can be observed only at high probe laser intensities. The resultant spectra exhibit marked power broadening and a pronounced dip appears in the center of each of the power broadened resonances. The observed line shapes are rationalized in terms of the spatial and temporal distributions of the probe laser field in conjunction with a dynamical competition between photoionization and spontaneous decay channels in the K state. Such arguments lead to an upper bound on the K 2Π(v=2) lifetime of ≊2 ns. The short lifetime of K 2Π(v=2) is attributed to indirect heterogeneous predissociation through the (4pσ)M 2Σ+ state and/or to homogeneous predissociation via the a 4Π continuum. [ABSTRACT FROM AUTHOR]

A systematic explanation of the pressure-dependent multiphoton ionization spectra of argon is given in terms of the phase matching requirements of third-harmonic generation. Two-color sum-frequency generation in a highly dispersive medium and its effects on normal three-photon excitation process are analyzed, using argon as an example. Finally an anomaly in the resonance line shapes of the two laser MPI spectra of argon is reported and attributed to the displacement of the foci of the two counterpropagating laser beams. [ABSTRACT FROM AUTHOR]

Mass-resolved, two-photon resonant, three-photon ionization of jet-cooled Cl2 has been used to study the spectroscopy and photophysics of the lowest energy, double-well 1Πg and 3Πg states of mixed Rydberg (4sσ) and ion-pair character. Inner well levels from v’=0 (64 027 cm-1) to v’=15 (73 343 cm-1) have been observed for 1Πg of the mass 70 molecular chlorine isotope. Inner well levels for 3Πg have been observed from v’=0 at 63 472 cm-1 to the top of the barrier, and well beyond that to levels spanning both wells. Observation of these long progressions is attributed to excitation via a repulsive continuum at a well-phonon level. The photophysics of this latter process as well as the mechanism for copious atomic ion formation are discussed. Variation of linewidth with vibrational quantum number due to the predissociation of inner well levels is observed and attributed to repulsive valence states. The data are in good agreement with the theoretical calculations of Peyerimhoff and Buenker, and Tuckett and Peyerimhoff. [ABSTRACT FROM AUTHOR]

The photoelectron spectrum (PES) of the E 3Σ-u state of O2 has been measured with 3+1 multiphoton ionization (MPI). The observed vibrational progression in the PE spectrum is interpreted as due to ionization from a (3pπ)3Σ-u Rydberg state which is strongly mixed with the (πu 3π3g)B 3Σ- valence state. The observation of this mixing is in near agreement with the calculation by Buenker et al. [ABSTRACT FROM AUTHOR]

We present a study of the multiphoton ionization spectrum of chlorine in the region of 2 1Πu←X 1Σ+g(1,0–0) three-photon resonances. Spectra at various pressures were obtained with different polarizations and counterpropagating beam experiments were carried out to restore the peaks quenched by third harmonic interference effects. Chlorine atoms, caused by photodissociation, produce prominent lines in the spectrum and are also responsible for third harmonic generation (THG) which in turn excites some spin-forbidden transitions by one-photon absorption. We discuss the change of polarization behavior in the presence of THG and the resultant effect on the symmetry assignments of the excited states. We also discuss third harmonic destructive interference effects in molecular systems and show that total suppression of three-photon transition to one-photon allowed states, common in atoms, will not usually occur in molecules although in general changes in polarization behavior should be observed at higher pressures. This conclusion is demonstrated in the chlorine spectrum. [ABSTRACT FROM AUTHOR]

Anomalous dips are observed in time-delayed, ionization-detected optical–optical double resonance (OODR) experiments on molecular nitrogen. These dips arise from the competition between radiative decay and photoionization of the upper level of the probe transition, and are therefore of considerable potential importance in the application of ionization-detected OODR to molecules. [ABSTRACT FROM AUTHOR]

Photoelectron spectra produced by (2+1) MPI via the v=0–3 levels of the (3sσg ) C 3 Πg Rydberg state of O2 show large deviations from the Franck–Condon principle. These deviations are attributed largely to a shape resonance in the σu ionization channel and probably a ‘‘window’’ resonance in the 3 Σ-u part of the πu channel. [ABSTRACT FROM AUTHOR]