The quantum yield for the formation of HCN from the photodissociation of pyrazine excited at 248 nm and 266 nm is determined by IR diode probing of the HCN photoproduct. HCN photoproducts from excited pyrazine are produced via three different dissociation channels, one that is extremely “prompt” and two others that are “late.” The total quantum yield from all reaction channels obtained at low quencher gas pressures, φ=1.3±0.2 for 248 nm and 0.5±0.3 for 266 nm, is in agreement with preliminary studies of this process as well as recent molecular beam studies. To investigate if HCN production is the result of pyrazine multiphoton absorption, this photodissociation process has been further studied by observing the HCN quantum yield as a function of total quencher gas pressure (10 mTorr pyrazine, balance SF6) and as a function of 248 nm laser fluence from 2.8 to 82 mJ/cm2. At the highest SF6 pressures, the HCN quantum yield shows strong positive correlation with laser fluence, indicating that the “prompt” channel is the result of multiphoton absorption; however, at low pressure, the HCN quantum yield is affected little by changing laser fluence, indicating that the majority of the HCN photoproducts at low pressure are produced from pyrazine which has absorbed only one UV photon. At the lowest pressures sampled, HCN produced from the one-photon “late” process accounts for more than 95% of all HCN formed (at low laser fluence). At high pressures the single photon “late” pyrazine dissociation is quenched, and HCN produced at high quencher gas pressures comes only from the multiphoton absorption channel, which can be clearly observed to depend on laser fluence. The HCN quantum yield as a function of laser intensity at high pressure has been fit to a quadratic function that can be used to determine the amount of “prompt” “unquenched” HCN produced from multiphoton photodissociation. Additionally, the information theoretic prior functions for energy disposal in the 248 nm photodissociation of pyrazine to form HCN have also been developed. Prior functions for one, two, and three-photon absorption indicate that only HCN with near room temperature translational energy comes from the one-photon process and that all HCN molecules with large amounts of translational energy are produced by multiphoton processes. Finally, analysis of the quenching data within the context of a strong collision model allows an estimate of the rate constant for HCN production from pyrazine for the major “late” channel, kd1s=1.69×105 s−1, for 248 nm excitation, and kd1s=1.33×104 s−1 for 266 nm excitation. After 266 nm excitation, pyrazine produced by the major one-photon channel lives for almost an order of magnitude longer than after 248 nm excitation.

Ambientdetection of NO2 by cavity ring-down spectroscopy is examined in the wavelength region near 405.23 nm, and possible interferences by particulates, water vapor, and carbon dioxide are characterized. Particulates can be efficiently removed by the use of a 0.45 microm fluoropolymer filter. Water vapor has a response of 2.8 ppb (NO2 equivalent) for 1.0% water vapor (80% relative humidity at 10 degrees C) in air at 405.23 nm in a broad continuous absorption feature. Carbon dioxide has a response of 0.8 ppb (NO2 equivalent) for 1.0% CO2 attributable to Rayleigh scattering and would not contribute significant interference in ambient measurements due to the lower ambient CO2 levels. Water vapor interference and in general broad background in the absorption spectrum can be accounted for by removing NO2 selectively in the ambient air stream with an annular denuder coated with sodium hydroxide and methoxyphenol (guiacol). Subtraction of the resulting background signal provides NO2 measurements with a limit of detection of 150 ppt/10 s (SIN = 3). Reliable NO2 measurements could be obtained by this method without the need for frequent calibration with calibration gas. Ambient NO2 measurements are carried out to demonstrate this method.

Our application of LabVIEW(R) software for computer data-acquisition using several techniques used across our curriculum is described. The techniques are gas chromatography, calorimetry, titrations and other volume-dependent techniques, spectrometry for kinetics using a Spectronic 20(R), and emission spectroscopy. Applications of these techniques range from general chemistry to physical chemistry, instrumental analysis, and undergraduate research. The hardware in common for all of the techniquesis a 4-1/2 digit multimeter connected to the computer via a GPIB interface to a Macintosh computer. Figures include examples of a LabVIEW "virtual instrument" (VI) diagram, some student-generated data, and a LabVIEW VI front panel.

The quantum yield for HCN formation via 248 and 266 nm photodissociation of methylpyrazine (C 5 N 2 H 6 ) is determined by IR diode probing. HCN is produced at two different dissociation rates, one of which is extremely prompt. The total quantum yield is φ ) 0.93 ( 0.08 for 248 nm and 0.35 ( 0.05 for 266 nm excitation. Analysis of the quenching data within the context of a gas kinetic, strong collision model allows an estimate of the rate constant for HCN production via "late" methylpyrazine photodissociation, k d1s ) 6.4 × 10 4 s -1 and k d1s ) 4.9 × 10 3 s -1 for 248 and 266 nm excitation, respectively. The rate constant for "prompt" dissociation is too large to be measured using this technique. After 266 nm excitation methylpyrazine lives more than an order of magnitude longer than after 248 nm excitation. Methylpyrazine also lives more than twice as long as pyrazine excited under identical conditions. Transient absorption measurements probing rotationally and translationally excited CO 2 molecules produced following excitation of methylpyrazine are analyzed within the context of a kinetic scheme incorporating methylpyrazine photodissociation, as well as excitation of CO 2 by both translationally hot HCN and vibrationally excited methylpyrazine. This analysis indicates that vibrationally hot methylpyrazine, which has sufficient energy to dissociate, is the source of excitation in collisions imparting large amounts of rotational and translational energy to CO 2 . † Part of the special issue "C. Bradley Moore Festschrift".

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