Multiplex spectroscopy of stable and transient species in a molecular beam

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 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 4051-Å band of C₃. 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 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.
Molecular beam apparatus
Multiplex spectra of the 4051-Å band of C₃

Photo-Fragment Excitation Spectroscopy (PHOFEX) by DFWM and LIF: Propensities for H₂CO -> HCO + H Near the S_o Threshold
A novel technique for photo-fragment excitation spectroscopy (PHOFEX) is introduced by applying resonantly enhanced degenerate four-wave mixing (DFWM) and laser-induced fluorescence (LIF). The method is applied to investigate the dynamics of the radical channel decomposition of H₂CO. The DFWM configuration is used to excite H₂CO to an energy level close to the threshold for dissociation to radical products on the S₀ potential energy surface. Simultaneously, laser induced fluorescence (LIF) is employed to detect the nascent HCO fragments. PHOFEX spectra are recorded by tuning the DFWM photon energy near the dissociation threshold and measuring specific HCO transitions by LIF. The PHOFEX spectra are subsequently normalized to the DFWM signals recorded simultaneously. The resulting fluorescence quantum yield spectra provide sensitive information on state-specific propensities of the dissociation reaction.

Stimulated Emission Pumping by Two-Color Resonant Four-Wave Mixing: Rotational Characterization of Vibrationally Excited HCO
Stimulated emission pumping by applying two-color resonant four-wave mixing is used to measure rotationally resolved spectra of the HCO (0,0,0)  B - (0,3,1) X transition. The formyl radical is produced by photodissociation of formaldehyde at 31710.8 cm ^-1 under thermalized conditions in a low pressure cell. In contrast to the highly congested one-color spectrum of HCO at room temperature, the double-resonance method yields well isolated transitions which are assigned unambiguously due to intermediate level labeling. 89 rotational transitions have been assigned and yield accurate rotational constants for the vibrationally excited (0,3,1) band of the electronic ground state of HCO. The determined rotational constant A = 25.84 0.01 cm^-1 is considerably higher than that for the vibrationless ground state and reflects the structural change due to excitation of the bending mode of the formyl radical.

Two-Color Resonant Four-Wave Mixing as a New Tool to Study State-to-State Energy Transfer.
Applying a recently developed ns Two-Colour Resonant Four-Wave Mixing (TC-RFWM) technique, a number of resonance lines forbidden by conventional three-level schemes are observed. A theoretical frequency-domain picture is developed with the help of exact (off-diagonal) relaxation matrices and general four-level schemes. The newly observed extra resonances are induced by collisional transfer of rotational energy. The role of the population, orientation and alignment gratings that are formed in both electronic states coupled by laser fields is elucidated. Relaxation models using the fast-collision approximation are elaborated for Hund's cases a and b giving quantitative arguments for the proposed interpretation. The potential of the TC-RFWM technique to study state-to-state transfer rates is emphasized.
 

Stimulated Emission Pumping of OH and NH in Flames by Using Two-Color Resonant Four-Wave Mixing.
In this work we examine the analytical potential of two-color resonant four-wave mixing for the determination and characterization of trace elements in a combustion environment. Experimental results for NH and OH in flames at atmospheric pressure are presented. The selectivity of the technique is used to simplify the Q-branch region of the (0-0) A - X vibronic transition of NH. Furthermore, substantial signal-to-noise ratios in the (0-0) A-X system of OH is achieved. The high sensitivity is applied to perform stimulated emission pumping involving the weak (0-1) vibrational band. In addition, we demonstrate that the technique is sensitive to state changing collisions.