A new apparatus is described that uses nonlinear and linear spectroscopic techniques simultaneously for the characterization of stable and transient molecules in a molecular beam environment. Short-lived species are generated by applying a well-defined discharge on a suitable precursor prior to supersonic expansion. Femtosecond ionization and mass spectrometry is used to optimize the discharge source. Degenerate and two-color resonant four-wave mixing spectroscopy (DFWM and TC-RFWM, respectively) are used in tandem with laser-induced fluorescence (LIF) and cavity ring-down spectroscopy (CRD). We demonstrate initial experiments on the 4051Å band of C 3 (Ã 1 5 u −X 1 6 + g , 000-000). DFWM, LIF and CRD display similarities in the general shape and position of the rovibronic transitions. A more detailed view, however, reveals the complementary character of the methods and the potential of simultaneously measured spectra. High signal-to-noise ratios and well-structured spectra are obtained by DFWM. LIF is favorable for the observation of weak features but strongly dependent on the relaxation mechanism of the upper excited state. CRD suffers generally from a large background signal but yields quantitative information. By applying the methods in parallel, quantitative measurements of molecular number densities, transitions dipole moments and relaxation rates are accessible.

Degenerate and two-color resonant four-wave mixing techniques (DFWM and TC-RFWM) are applied to determine rotational constants of high-lying vibrational levels in the first excited singlet stateÃ( 1 A 2 ) of formaldehyde. It has been demonstrated that the sensitivity of the spectroscopic technique is applicable to the low-density environment of a supersonic molecular beam and to predissociating transitions displaying low fluorescence quantum yield. In addition, we take advantage of the superior selectivity of the doubleresonance method, TC-RFWM, to isolate and assign transitions in the congested region of the (one-color) DFWM spectra. The line positions of 25 well-isolated transitions are determined in the 2 2 0 5 1 0 band and yield the rotational constants A, B, C and the origin n e . The accuracy of the constants is determined by performing the same procedure for the 2 2 0 4 1 0 band where literature data is available for comparison.

A comparison of degenerate four-wave mixing (DFWM) and cavity ring-down (CRD) spectroscopy is made on jet-cooled formaldehyde under identical expansion conditions. We show that both methods are applicable to the low-density environment of a molecular beam. However, a slightly superior signalto-noise ratio for the background-free DFWM method is observed for our experimental configurations. Absorption spectra simulated by applying an asymmetric rotor model are used to deduce Boltzmann temperatures of the rotational population distribution by a nonlinear least-squares fit to the experimental data. In addition, partial interconversion of the nuclear spin upon supersonic expansion is observed. Consequently, a second fit parameter, describing the para/ortho concentration ratio in the molecular beam, is introduced. The resulting simulated spectra are in favorable agreement with the relative CRD intensities. A simple model for the reduction of the DFWM signal intensities to population is applied yielding absorption-like spectra in accordance with both the simulations and the CRD measurements. We conclude that DFWM spectroscopy is applicable to deduce population distributions of an asymmetric rotor molecule under free-jet conditions.

The rotational structure of the lowest three vibrational levels (0(0), 6(1) and 4(1)) of the 3p(x) (1)A(2) Rydberg state of formaldehyde has been studied by doubly-resonant three-photon ionization spectroscopy. A strong a-type Coriolis interaction between the in-plane rocking (ν(6)) and out-of-plane bending (ν(4)) modes results in the observation of vibronically forbidden transitions to the 6(1) level from the intermediate Ã(1)A(2) (2(1) 4(3)) level. The full widths at half maximum of the rovibronic transitions to the 4(1) state are considerably larger than to the vibrational ground state and the 6(1) level. The band origin (T(0) = 67 728.939(82) cm(-1)), the rigid rotor rotational constants (A = 9.006(19) cm(-1), B = 1.331(20) cm(-1), and C = 1.135(22) cm(-1)), the Coriolis coupling constant (ξ = 8.86(89) cm(-1)) and the deperturbed fundamental wave numbers of both vibrational modes (v[combining tilde](6) = 808.88(25) cm(-1) and v[combining tilde](4) = 984.92(26) cm(-1)) have been determined for the 3p(x) (1)A(2) Rydberg state. Polarization effects originating from the double-resonance technique have been exploited to detect the Coriolis interaction and investigate how it affects the predissociation dynamics.

The first electronically excited singlet A 1A2 state of formaldehyde (H2CO) is the origin of different competing dissociation channels. An all-ultraviolet (UV) electronically resonant variant of time-resolved fs-two-colour four-wave mixing (fs-TCFWM) spectroscopy has been applied to monitor the dynamics of rovibrational manifolds in this state. The method provides a variety of options to probe molecular dynamics according to the temporal sequence of dipole transitions involved. Signal contributions arising from different molecular states can partially be distinguished by specific sequences of the applied laser pulses. A dispersed signal detection method reveals additional information that is often inaccessible using spectrally integrated detection. We discuss UV-fs-TCFWM experiments which involve resonant transitions to the rovibrational manifolds of the C-O stretch and out-of-plane combination bands in the first electronically excited singlet state of H2CO at energies close to the HCO production threshold. The experimental results are compared to simulations.