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Methyl radicals are generated in a hot-filament diamond synthesis reactor using a resistively heated tungsten filament (20 mm long) in a slowly flowing mixture of 0.5% CH 4 in H 2 . The UV absorbance of CH 3 is measured during deposition using a line-of-sight optical technique called cavity ring-down spectroscopy (CRDS). Measurements are carried out at 213.9 nm, a wavelength at which the CH 3 absorption cross section has been shown, by others, to be independent of temperature over a large range. We observe a strong sensitivity of the methyl radical concentration throughout the reactor to the substrate temperature. At some operating conditions, we also observe the methyl radical concentration to peak at a location several millimeters from the filament surface. This behavior of CH 3 with distance from filament is in qualitative agreement with two-dimensional models of the deposition environment, and is attributed to the effect of Soret diffusion on the balance of the primary methyl production/destruction reaction. more...

We have investigated the implications of using a pulsed, nearly Fourier-transform-limited, single-mode light source for cavity ring-down spectroscopy (CRDS) in the mid-infrared spectral range. We show that in the case where the coherence time and duration of the light pulse exceeds the cavity roundtrip time, mode beating generates oscillations in the ring-down waveform. When the period of the oscillations is comparable to the ring-down time, it becomes difficult to obtain meaningful decay constants. This situation can be avoided by careful choice of cavity geometry and mode matching conditions together with suitable electronic filtering. more...

Methyl radicals were generated in a hot-filament diamond synthesis reactor using a resistively heated tungsten filament (length, 20 mm) in a slowly flowing mixture of 0.5% CH 4 in H 2 . The UV absorbance of CH 3 was measured during deposition using a new line-of-sight optical technique called cavity ring-down spectroscopy (CRDS). Measurements were carried out at 213.9 nm, a wavelength at which the CH 3 absorption cross-section has been shown by others to be independent of the temperature over a large range. We observed that the CH 3 absolute concentration varied strongly as a function of the position between the substrate and the filament, and its value was strongly influenced by the substrate temperature. more...

Cavity ring-down spectroscopy has been used to measure the absorbance of methyl radicals near 216 nm. The methyl radicals are generated by a hot tungsten filament heated to 2300 K in a mixture of 0.5% CH 4 in H 2 slowly flowing through the reactor at 20 Torr total pressure. CH 3 absorbances are detected with a noise-equivalent sensitivity of two parts in 10 5 using a narrow pencil of UV light 0.5 mm in diameter, which allows measurement of spatial profiles of CH 3 for column densities of 3×10 12 radicals/cm 2 (3×10 12 radicals/cm 3 ×1 cm absorption pathlength). more...

In this paper we analyse the effect of coherent saturation in the laser-induced fluorescence (LIF) of molecules. We develop an earlier model introduced by Billy et al. in 1987 of this coherent effect and propose a simple formula predicting the line shape and the line intensity for a laser-molecule interaction time considerably longer than the excited-state radiative lifetime. The predictions of this formula are tested on a LIF experiment in which a Na2 molecular beam is excited with a continuous-wave single-frequency dye laser. Considerable spectral broadening, up to two hundred times the natural linewidth is observed, in good agreement with the model. more...

We report observation of cluster formation initiated by laser light in a cell containing sodium vapor and a buffer gas (He) under high pressure (550 Torr). Switching on the laser light induces the process of cluster condensation, which develops spontaneously; changes in size and density of transient clusters are observed lasting from ten to twenty seconds after the light is switched on. more...

The spatial profile of methyl radical concentration in a hot‐filament reactor has been measured using cavity ring‐down spectroscopy (CRDS) at a wavelength of 213.9 nm for which the CH3 absorption cross section has been shown to be nearly independent of temperature. Methyl radicals are generated with a 25 mm long tungsten filament heated to 2400 K in a slowly flowing mixture of 0.6% CH4 in H2 (20 Torr total pressure). CRDS is employed to measure CH3 absorbance as a function of a distance perpendicular to the axis of the filament. The CH3 absorbance profiles do not change when the direction of the process gas flow through the reactor is reversed, which indicates cylindrical symmetry of the CH3 distribution about the filament. Consequently, the radial CH3 concentration in the reactor is determined by Abel inversion of the CH3 absorbance profile. The CH3 concentration peaks ∼4 mm from the filament (1.04×1014 molecules/cm3). Methyl radicals decay rapidly as a function of a distance from the filament and disapp... more...

The lowest electronically excited states of Na2 are of interest as intermediaries in the excitation of higher states and in the development of methods for producing cold molecules. We have compiled previously obtained spectroscopic data on the AΣu+1 and bΠu3 states of Na2 from about 20 sources, both published and unpublished, together with new sub-Doppler linewidth measurements of about 15 000 A←X transitions using polarization spectroscopy. We also present new ab initio results for the diagonal and off-diagonal spin-orbit functions. The discrete variable representation is used in conjunction with Hund’s case a potentials plus spin-orbit effects to model data extending from v=0 to very close to the 3S2+3P1∕22 limit. Empirical estimates of the spin-orbit functions agree well with the ab initio functions for the accessible values of R. The potential function for the A state includes an exchange potential for S+P atoms, with a fitted coefficient somewhat larger than the predicted value. Observed and calculat... more...