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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 So 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.