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In this work we present a deperturbation study of the d 3Πg,v=6 state of C2 by double-resonant four-wave mixing spectroscopy. Accurate line positions of perturbed transitions are unambiguously assigned by intermediate level labeling. In addition, extra lines are accessible by taking advantage of the sensitivity and high dynamic range of the technique. These weak spectral features originate from nearby-lying dark states that gain transition strength through the perturbation process. The deperturbation analysis of the complex spectral region in the (6,5) and (6,4) bands of the Swan system (d3Πg-a 3Πu) unveils the presence of the energetically lowest high-spin state of C2 in the vicinity of the d 3Πg,v=6 state. The term energy curves of the three spin components of the d state cross the five terms of the 1 5Πg state at rotational quantum numbers N <= 11. The spectral complexity for transitions to the v = 6 level of d 3Πg state is further enhanced by an additional perturbation at N = 19 and 21 owing to the b 3Σg-,v=19 state. The spectroscopic characterization of both dark states is accessible by the measurement of 122 'window' levels. A global fit of the positions to a conventional Hamiltonian for a linear diatomic molecule yields accurate molecular constants for the quintet and triplet perturber states for the first time. In addition, parameters for the spin-orbit and L-uncoupling interaction between the electronic levels are determined. The detailed deperturbation study unravels major issues of the so-called high-pressure bands of C2. The anomalous nonthermal emission initially observed by Fowler in 1910 [Mon. Not. R. Astron. Soc. 70, 484 (1910)] and later observed in numerous experimental environments are rationalized by taking into account 'gateway' states, i.e., rotational levels of the d 3Πg,v=6 state that exhibit significant 5Πg character through which all population flows from one electronic state to the other. [ABSTRACT FROM AUTHOR]

We show that the technique of femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy provides a powerful tool for the investigation of collision-induced linewidths and the validation of rotational energy transfer (RET) models. The fs-CARS method is applied to the N[sub 2]–N[sub 2] collision system, and a comparison between the commonly used exponential gap (ECS-E), power gap (ECS-P), frequency corrected (EFCS), and the recently proposed angular momentum and energy corrected (AECS) variants of the ECS model is presented. As result we show that the AECS scaling law requires only two free parameters, and is appropriate for the determination of RET rates from the measured fs-CARS signals. The AECS model is also applied to the more complex C[sub 2]H[sub 2]–C[sub 2]H[sub 2] collision system. As vibrational energy transfer and dephasing is not negligible in this case, the model has to be modified by introducing a vibrational relaxation factor. With this modification the fs-CARS signals from acetylene can be described successfully. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Knopp et al. [J. Raman Spectrosc. 31, 51 (2000)] have recently used resonant femtosecond coherent anti-Stokes Raman spectroscopy (CARS) to prepare and probe highly excited vibrational wave packets on the ground electronic potential surface of molecular iodine. The experiment uses a sequence of three resonant femtosecond pulses with two independently variable time delays. The first two pulses act as a pump and dump sequence to create a predefined, highly excited wave packet on the ground electronic state, whose amplitude is optimized by selecting the proper pump–dump (Raman) frequency difference and varying the time delay. The third pulse promotes the pump–dump wave packet to an excited electronic state, resulting in subsequent coherent emission of light at the anti-Stokes frequency. This fully-resonant CARS signal, measured as a function of time delay between the second and third pulses, oscillates at a frequency characteristic of the pump–dump wave packet. Due to anharmonicity, this frequency is a sensitive measure of the amount of vibrational excitation. Knopp et al. observed that under certain conditions the signal exhibits pronounced beating between the pump–dump wave packet frequency and the frequency characteristic of the bottom of the ground state well. In this paper we show that these beats arise only when the final pump–dump–pump wave packet is above the excited state dissociation threshold of the molecule. We derive analytical expressions showing that under these conditions, where the polarization is short-lived, there may be strong interferences between the contributions from molecules originally in different vibrational states of the thermal ensemble. In contrast, the CARS polarization in the below threshold case is long-lived, and these interferences cancel. Numerical evaluation of the CARS signal through vibrational wave packet propagation confirms the predictions of the analytical theory and reproduc... [ABSTRACT FROM AUTHOR]

The potential of two-color resonant and degenerate four-wave mixing spectroscopy for investigations of the complex spectra of transition metal dimers is explored. Two-color resonant and degenerate four-wave mixing spectroscopy scans of the well-known A-X and B-X transitions in Cu2 are reported and compared with previous experimental data obtained from standard single-resonance techniques. The selectivity of the method is shown to enable the measurement of isotopologue pure spectra without the need for isotopically enriched metal targets. Specific subsets of the rovibronic structure are separated in a congested spectral region of overlapping transitions. The sensitivity of the method compares satisfactorily with linear spectroscopic methods such as laser-induced fluorescence and cavity ring-down. A new laser vaporization source for the production of transition metal dimers and clusters has been constructed. The new design aims for a high number density and maximum possible shot-to-shot stability. The possibilities of further applications of non-linear four-wave mixing spectroscopy to Cu2 and other transition metal dimers are discussed. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The recombination dynamics and ultrafast dissociation of I(sub 2) enclosed in well-defined cages are investigated using femtosecond pump-probe spectroscopy. The cages, each of which is occupied by one guest molecule corresponding to a high density of I(sub 2), are formed by the electroneutral host framework of the crystalline microporous SiO2 modification decadodecasil 3R. It is shown that collision-induced predissociation occurs after excitation into the bound B state, followed by recombination into the A and A' states. The molecules undergo vibrational relaxation in these states.