Michel J. Rossi

Reaction of Chlorine Nitrate with Hydrogen Chloride and Water at Antarctic Stratospheric Temperatures

Laboratory studies of heterogeneous reactions important for ozone depletion over Antarctica are reported. The reaction of chlorine nitrate (ClONO2) with H20 and hydrogen chloride (HCl) on surfaces that simulate polar stratospheric clouds [ice and nitric acid (HNO3)—ice and sulfuric acid] are studied at temperatures relevant to the Antarctic stratosphere. The reaction of ClONO2 on ice and certain mixtures of HNO3 and ice proceeded readily. The sticking coefficient of ClONO2 on ice of 0.009 � 0.002 was observed. A reaction produced gas-phase hypochlorous acid (HOCl) and condensed-phase HNO3; HOC1 underwent a secondary reaction on ice producing dichlorine monoxide (Cl2O). In addition to the reaction with H20, ClONO2 reacted with HCl on ice to form gas-phase chlorine (Cl2) and condensed-phase HNO3. Essentially all of the HCl in the bulk of the ice can react with ClONO2 on the ice surface. The gaseous products of the above reactions, HOCl, Cl20, and Cl2, could readily photolyze in the Antarctic spring to produce active chlorine for ozone depletion. Furthermore, the formation of condensed-phase HNO3 could serve as a sink for odd nitrogen species that would otherwise scavenge the active chlorine.

Energy‐dependent energy transfer: Deactivation of azulene (S0, Evib) by 17 collider gases

Collisional deactivation of highly vibrationally excited azulene in the electronic ground state was investigated using infrared fluorescence detection. Azulene (S0, E) was prepared with E≂17 500 cm−1 and E≂30 600 cm−1 by laser excitation at 600 and 337 nm, respectively. Advantage was taken of the fast internal conversion rate to S0 azulene from S1(600 nm) and S2(337 nm) electronic states. The collider gases investigated are He, Ne, Ar, Kr, Xe, H2, D2, N2, CO, O2, CO2, H2O, NH3, CH4, SF6, n‐C4H10, and unexcited azulene. The results are expressed in terms of 〈ΔE(E)〉, the average energy transferred per collision, which can depend on the vibrational excitation energy E of the azulene. Using previously obtained knowledge of the dependence of infrared fluorescence intensity on E [M. J. Rossi and J. R. Barker, Chem. Phys. Lett. 85, 21 (1982)], two methods were used to obtain 〈ΔE(E)〉 values from the fluorescence decay curves: (1) an approximate method that considered only the average energy, and (2) solution of t...

Antarctic Ozone Depletion Chemistry: Reactions of N2O5 with H2O and HCl on Ice Surfaces

The reactions of dinitrogen pentoxide (N2O5) with H2O and hydrochloric acid (HCl) were studied on ice surfaces in a Knudsen cell flow reactor. The N2O5 reacted on ice at 185 K to form condensed-phase nitric acid (HNO3). This reaction may provide a sink for odd nitrogen (NOx) during the polar winter, a requirement in nearly all models of Antarctic ozone depletion. A lower limit to the sticking coefficient, γ, for N2O5 on ice is 1 x 10-3. Moreover, N2O5 reacted on HCl-ice surfaces at 185 K, with γ greater than 3 x 10-3. This reaction, which produced gaseous nitryl chloride (ClNO2) and condensed-phase HNO3, proceeded until all of the HCl within the ice was depleted. The ClNO2, which did not react or condense on ice at 185 K, can be readily photolyzed in the Antarctic spring to form atomic chlorine for catalytic ozone destruction cycles. The other photolysis product, gaseous nitrogen dioxide (NO2), may be important in the partitioning of NOx between gaseous and condensed phases in the Antarctic winter.

Infrared multiphoton generation of radicals: A new technique for obtaining absolute rate constants. Application to reactions of CF3

The infrared multiphoton dissociation of CF3I is used as a convenient source of CF3 radicals under very low pressure conditions. Measurement of thermal absolute rate constants at 298 K for the reactions CF3+Br2→CF3Br+Br, CF3+ClNO→CF3Cl+NO, CF3+O3→CF3O+O2, and CF3+NO2→CF3O+NO yield (7.8±1.3) ×108, (3.5±0.5) ×108, (5.6±0.8) ×108, and (1.6±0.3) ×109 M−1 s−1, respectively. This new technique shows great promise for production of other free radicals of interest and measurement of thermal absolute rate constants over a wide temperature range.

Infrared fluorescence and collisional energy transfer parameters for vibrationally excited azulene*(So): dependence on internal energy (Evib)

Abstract Infrared fluorescence (IRF) from the CH stretch modes of vibrationally excited gas-phase azulene * was observed to depend on E vib according to a simple model. The IRF time/pressure behavior shows that the average energy transferred per collision depends strongly on E vib for azulene * + azulene and azulene * + argon collisions.

Infrared multiphoton dissociation yields via a versatile new technique: intensity, fluence, and wavelength dependence for CF3I

Abstract Intensity, fluence, and wavelength dependence of the collisionless infrared multiphoton dissociation yield for CF 3 I have been determined via “titration” of radical photoproducts and molecular-beam-sampling-mass spectrometry. The importance of nonlinear effects in the pumping of this molecule is confirmed.

Absolute rate of recombination of CF3 radicals

Abstract The homogeneous rate of recombination of CF 3 radicals at 300 K was studied in a very-low-pressure photolysis flow reactor. The radicals were generated from CF 3 I by either CO 2 laser (1076 cm −1 or excimer laser (248 nm) photolysis. The precursor and product concentrations were followed by mass spectrometry. The recombination rate constant, K r , was measured as (3.0±0.4)×10 −12 cm 3 s −1 . A hindered Gorin transition-state model predicts that k r /k ∞ r is only 0.74 at the effective pressures employed. The model was used to fit these experiments at 300 K, as well as earlier experiments at ≈ 1000 K.

Photoenhanced electron attachment of vinylchloride and trifluoroethylene at 193 nm

We show that the electron attachment properties of a gas mixture of helium containing vinylchloride or trifluoroethylene can be altered from nonattaching to strongly attaching by irradiation with a low‐energy laser pulse at 193 nm. These molecules are photodissociated producing vibrationally excited HCl or HF and other fragments that strongly attach low‐energy electrons.

Atom- and radical-surface sticking coefficients measured using resonance-enhanced multiphoton ionization

Sticking coefficients γ of neutral transient species at ambient temperature were measured using in situ resonance enhanced multiphoton ionization (REMPI) of the transients in a low pressure reactor at mTorr pressure. The value of γ for I on a stainless steel surface was 0.16, whereas γ for CF3 free radical on the same surface was 0.5 was found for highly vibrationally excited CF3 containing 5900 cm−1 of internal energy and for SiH2 containing 7000 cm−1 of internal energy. The surface was stainless steel in the former case and a carbon‐containing Si and H surface in the latter case.

Level-to-level vibrational energy transfer studies: energy dependence and observation of product species for azulenes(S0, Evib) + CO2☆

V—V energy transfer from a large molecule excited to vibrational energies of chemical interest has been demonstrated by detection of ≈ 1.5% yield of CO2(001) due to energy transfer from azulene (Evib ≈ 30600 cm−1. Also, the average enery lost per collision by azulene was measured as a function of Evib, and the rate constant for CO2(001) deactivation by azulene was determined.