Detection of vibration-rotational band 5-0 of 12C16O X 1sigma+ with cavity ringdown absorption near 0.96 microm

We have recorded extremely weak absorption in the overtone band 5-0 of 12C16O X 1sigma+ near 0.96 microm with cavity ringdown spectroscopy; the light source was a Raman-shifted dye laser pumped with a frequency-doubled Nd:YAG laser. This band shows lines in branch P to be much more intense than corresponding lines in branch R, in contrast to all lower overtone bands v-0 (v = 1-4). This reversal in relative intensity is explained quantitatively in terms of a radial function for the electric dipolar moment of CO. We have estimated absorption line strengths for P3-P18 in band 5-0 of 12C16O; these strengths are consistent with a pure vibrational matrix element <5/p(x)/0> = (3.6 +/- 0.3) x 10(-36) C m of the electric dipolar moment p(x), a Herman-Wallis coefficient C0(5) of about -0.1, and a band strength of (5.1 +/- 1.3) x 10(-29) m at 293 K.     

Site-selective nitrogen isotopic ratio measurement of nitrous oxide using 2 microm diode lasers

We demonstrate a high-precision measurement of the isotopomer abundance ratio 14N(15)N(16)O/15N(14)N(16)O/14N(14)N(16)O (approximately 0.37/0.37/100) using three wavelength-modulated 2 microm diode lasers combined with a multipass cell which provides different optical pathlengths of 100 and 1 m to compensate the large abundance difference. A set of absorption lines for which the absorbances have almost the same temperature dependence are selected so that the effect of a change in gas temperature is minimized. The test experiment using pure nearly natural-abundance N(2)O samples showed that the site-selective 15N/14N ratios can be measured relative to a reference material with a precision of +/-3 x 10(-4) (+/-0.3 per thousand) in approximately 2 h.     

Relaxation of H2O from its /04>- vibrational state in collisions with H2O, Ar, H2, N2, and O2

We report rate coefficients at 293 K for the collisional relaxation of H2O molecules from the highly excited /04>(+/-) vibrational states in collisions with H2O, Ar, H2, N2, and O2. In our experiments, the mid R:04(-) state is populated by direct absorption of radiation from a pulsed dye laser tuned to approximately 719 nm. Evolution of the population in the (/04>(+/-)) levels is observed using the combination of a frequency-quadrupled Nd:YAG laser, which selectively photolyses H2O(/04>(+/-)), and a frequency-doubled dye laser, which observes the OH(v=0) produced by photodissociation via laser-induced fluorescence. The delay between the pulse from the pump laser and those from the photolysis and probe lasers was systematically varied to generate kinetic decays. The rate coefficients for relaxation of H2O(/04>(+/-)) obtained from these experiments, in units of cm3 molecule(-1) s(-1), are: k(H2O)=(4.1+/-1.2) x 10(-10), k(Ar)=(4.9+/-1.1) x 10(-12), k(H2)=(6.8+/-1.1) x 10(-12), k(N2)=(7.7+/-1.5) x 10(-12), k(O2)=(6.7+/-1.4) x 10(-12). The implications of these results for our previous reports of rate constants for the removal of H2O molecules in selected vibrational states by collisions with H atoms (P. W. Barnes et al., Faraday Discuss. Chem. Soc. 113, 167 (1999) and P. W. Barnes et al., J. Chem. Phys. 115, 4586 (2001).) are fully discussed.     

Laser induced fluorescence studies of iodine oxide chemistry. Part II. The reactions of IO with CH3O2, CF3O2 and O3

The technique of pulsed laser photolysis was coupled to laser induced fluorescence detection of iodine oxide (IO) to measure rate coefficients, k for the reactions IO + CH(3)O(2)--> products (R1, 30-318 Torr N(2)), IO + CF(3)O(2)--> products (R2, 70-80 Torr N(2)), and IO + O(3)--> OIO + O(2) (R3a). Values of k(1) = (2 +/- 1) x 10(-12) cm(3) molecule(-1) s(-1), k(2) = (3.6 +/- 0.8) x 10(-11) cm(3) molecule(-1) s(-1), and k(3a) <5 x 10(-16) cm(3) molecule(-1) s(-1) were obtained at T = 298 K. In the course of this work, the product yield of IO from the reaction of CH(3)O(2) with I was determined to be close to zero, whereas CH(3)OOI was formed efficiently at 70 Torr N(2). Similarly, no evidence was found for IO formation in the CF(3)O(2) + I reaction. An estimate of the rate coefficients k(CH(3)O(2) + I) = 2 x 10(-11) cm(3) molecule(-1) s(-1) and k(CH(3)OOI + I) = 1.5 x 10(-10) cm(3) molecule(-1) s(-1) was also obtained. The results on k(1)-k(3) are compared to the limited number of previous investigations and the implications for the chemistry of the marine boundary layer are briefly discussed.     

High precision measurements of atmospheric nitrous oxide and methane using thermoelectrically cooled mid-infrared quantum cascade lasers and detectors

A compact, fast response, mid-infrared absorption spectrometer using thermoelectrically (TE) cooled pulsed quantum cascade (QC) lasers and TE detectors has been developed to demonstrate the applicability of QC lasers for high precision measurements of nitrous oxide and methane in the earth's atmosphere. Reduced pressure extractive sampling with a 56 m path length, 0.5 l volume, multiple pass absorption cell allows a time response of <0.1s which is suitable for eddy correlation flux measurements for these gases. Precision of 0.3 ppb (rms, 1s averaging time) or 0.1% of the ambient concentration for N(2)O (4 ppb or 0.2% of ambient for CH(4)), has been demonstrated using QC lasers at 4.5 microm (7.9 microm for CH(4)), corresponding to an absorbance precision of 4 x 10(-5) Hz(-1/2) (8 x 10(-5) Hz(-1/2) for CH(4)). Stabilization of the temperature of the optical bench and the pulse electronics results in a minimum Allan variance corresponding to 0.06 ppb for N(2)O with an averaging time of 100 s (0.7 ppb with an averaging time of 200 s for CH(4)). The instrument is capable of long-term, unattended, continuous operation without cryogenic cooling of either laser or detector.     

State-resolved collisional quenching of vibrationally excited pyrazine (E(vib) = 37,900 cm(-1)) by D35Cl(v = 0)

Supercollision relaxation of highly vibrationally excited pyrazine (E(vib) = 37,900 cm(-1)) with D35Cl is investigated using high-resolution transient IR diode laser absorption spectroscopy at 4.4 microm. Highly excited pyrazine is prepared by pulsed UV excitation at 266 nm, followed by rapid radiationless decay to the ground electronic state. The rotational energy distribution of the scattered DCl (v = 0,J) molecules with J = 15-21 is characterized by T(rot) = 755+/-90 K. The relative translational energy increases as a function of rotational quantum number for DCl with T(rel) = 710+/-190 K for J = 15 and T(rel) = 1270+/-240 K for J = 21. The average change in recoil velocity correlates with the change in rotational angular momentum quantum number and highlights the role of angular momentum in energy gain partitioning. The integrated energy-transfer rate for appearance of DCl (v = 0,J = 15-21) is k(2)(int) = 7.1x10(-11) cm3 molecule(-1) s(-1), approximately one-eighth the Lennard-Jones collision rate. The results are compared to earlier energy gain measurements of CO2 and H2O.     

Determination of the rate constant for the OH(X2Pi) + OH(X()Pi) --> O(3P) + H2O reaction over the temperature range 293-373 K

The rate constant for the reaction OH(X2Pi) + OH(X2Pi) --> O(3P) + H2O has been measured over the temperature range 293-373 K and pressure range 2.6-7.8 Torr in both Ne and Ar bath gases. The OH radical was created by 193 nm laser photolysis of N2O to produce O(1D) atoms that reacted rapidly with H2O to produce the OH radical. The OH radical was detected by quantitative time-resolved near-infrared absorption spectroscopy using Lambda-doublet resolved rotational transitions of the first overtone of OH(2,0) near 1.47 microm. The temporal concentration profiles of OH were simulated using a kinetic model, and rate constants were determined by minimizing the sum of the squares of residuals between the experimental profiles and the model calculations. At 293 K the rate constant for the title reaction was found to be (2.7 +/- 0.9) x 10(-12) cm(3) molecule(-1) s(-1), where the uncertainty includes an estimate of both random and systematic errors at the 95% confidence level. The rate constant was measured at 347 and 373 K and found to decrease with increasing temperature.     

Kinetics of the radical-radical reaction, O(3P(J)) + OH(X2Pi omega) --> O2 + H, at temperatures down to 39 K

The kinetics of the reaction between O atoms and OH radicals, both in their electronic ground state, have been investigated at temperatures down to ca. 39 K. The experiments employed a CRESU (Cinétique deRéaction en Ecoulement Supersonique Uniforme) apparatus to attain low temperatures. Both reagents were created using pulsed laser photolysis at 157.6 nm of mixtures containing H2O and O2 diluted in N2 carrier gas. OH radicals were formed by both direct photolysis of H2O and the reaction between O(1D) atoms and H2O. O(3P) atoms were formed both as a direct product of O2 photolysis and by the rapid quenching of O(1D) atoms formed in that photolysis by N2 and O2. The rates of removal of OH radicals were observed by laser-induced fluorescence, and concentrations of O atoms were estimated from a knowledge of the absorption cross-section for O2 at 157.6 nm and of the measured fluence from the F2 laser at this wavelength. To obtain a best estimate of the rate constants for the O + OH reaction, we had to correct the raw experimental data for the following: (a) the decrease in the laser fluence along the jet due to the absorption by O2 in the gas mixture, (b) the increase in temperature, and consequent decrease in gas density, as a result of energy released in the photochemical and chemical processes that occurred, and (c) the formation of OH(v = 0) as a result of relaxation, particularly by O2, of OH radicals formed in levels v > 0. Once these corrections were made, the rate constant for reaction between OH and O(3P) atoms showed little variation in the temperature range of 142 to 39 K and had a value of (3.5 +/- 1.0) x 10(-11) cm3 molecule(-1) s(-1). It is recommended that this value is used in future chemical models of dense interstellar clouds.     

Reaction CH(3) + OH Studied over the 294-714 K Temperature and 1-100 bar Pressure Ranges

Reaction of methyl radicals with hydroxyl radicals, CH(3) + OH → products (1) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 294-714 K temperature and 1-100 bar pressure ranges (bath gas He). Methyl radicals were produced by photolysis of acetone at 193.3 nm. Hydroxyl radicals were generated in reaction of electronically excited oxygen atoms O((1)D), produced in the photolysis of N(2)O at 193.3 nm, with H(2)O. Temporal profiles of CH(3) were recorded via absorption at 216.4 nm using xenon arc lamp and a spectrograph; OH radicals were monitored via transient absorption of light from a dc discharge H(2)O/Ar low pressure resonance lamp at ca. 308 nm. The absolute intensity of the photolysis light inside the reactor was determined by an accurate in situ actinometry based on the ozone formation in the presence of molecular oxygen. The results of this study indicate that the rate constant of reaction 1 is pressure independent within the studied pressure and temperature ranges and has slight negative temperature dependence, k(1) = (1.20 ± 0.20) × 10(-10)(T/300)(-0.49) cm(3) molecule(-1) s(-1).     

Crystal structure, Judd-Ofelt analysis, and spectroscopic assessment of a TmAl3(BO3)4 crystal as a new potential diode-pumped laser near 1.9 micron

TmAl3(BO3)4 crystallizes in the trigonal system R32 (No. 155) with a = b = 9.2741(13) A, c = 7.218(3) A, alpha = beta = 90 degrees , gamma = 120 degrees , V = 537.7(2) A(3), D(c) = 4.494 g cm(-3), and Z = 3. The absorption spectrum of this crystal was recorded at room temperature. The Judd-Ofelt (J-O) theory was applied to the absorption intensities of TmAl(3)(BO3)4 to obtain the three J-O parameters: Omega(2) = 2.40 x 10(-20) cm(2), Omega(4) = 0.48 x 10(-20) cm(2), and Omega(6) = 1.09 x 10(-20) cm(2). The radiative probabilities, radiative lifetimes, and branching ratios of TmAl3(BO3)4 were calculated. The absorption and emission cross sections, together with the potential laser gain near 1.9 microm, were investigated. The potential laser gain curves indicate that the tunability range is about 200 nm.     

Falloff curves for the Reaction CH3 + O2 (+ M) --> CH3O2 (+ M) in the pressure range 2-1000 bar and the temperature range 300-700 K

The reaction CH(3) + O(2) (+M) --> CH(3)O(2) (+M) was studied in the bath gases Ar and N(2) in a high-temperature/high-pressure flow cell at pressures ranging from 2 to 1000 bar and at temperatures between 300 and 700 K. Methyl radicals were generated by laser flash photolysis of azomethane or acetone. Methylperoxy radicals were monitored by UV absorption at 240 nm. The falloff curves of the rate constants are represented by the simplified expression k/k(infinity) approximately [x/(1 + x)]F(cent)(1/{1+[(log)(x)/)(N)(]2}) with x = k(0)/k(infinity) F(cent) approximately 0.33, and N approximately 1.47, where k(0) and k(infinity) denote the limiting low and high-pressure rate constants, respectively. At low temperatures, 300-400 K, and pressures >300 bar, a fairly abrupt increase of the rate constants beyond the values given by the falloff expressions was observed. This effect is attributed to a contribution from the radical complex mechanism as was also observed in other recombination reactions of larger radicals. Equal limiting low-pressure rate constants k(0) = [M]7 x 10(-31)(T/300 K)(-3.0) cm(6) molecule(-2) s(-1) were fitted for M = Ar and N(2) whereas limiting high-pressure rate constants k(infinity) = 2.2 x 10(-12)(T/300 K)(0.9) cm(3) molecule(-1) s(-1) were approached. These values are discussed in terms of unimolecular rate theory. It is concluded that a theoretical interpretation of the derived rate constants has to be postponed until better information of the potential energy surface is available. Preliminary theoretical evaluation suggests that there is an "anisotropy bottleneck" in the otherwise barrierless interaction potential between CH(3) and O(2).     

Visible absorption spectrum of the CH3CO radical

The visible absorption spectrum of the acetyl radical, CH(3)CO, was measured between 490 and 660 nm at 298 K using cavity ring-down spectroscopy. Gas-phase CH(3)CO radicals were produced using several methods including: (1) 248 nm pulsed laser photolysis of acetone (CH(3)C(O)CH(3)), methyl ethyl ketone (MEK, CH(3)C(O)CH(2)CH(3)), and biacetyl (CH(3)C(O)C(O)CH(3)), (2) Cl + CH(3)C(O)H --> CH(3)C(O) + HCl with Cl atoms produced via pulsed laser photolysis or in a discharge flow tube, and (3) OH + CH(3)C(O)H --> CH(3)CO + H(2)O with two different pulsed laser photolysis sources of OH radicals. The CH(3)CO absorption spectrum was assigned on the basis of the consistency of the spectra obtained from the different CH(3)CO sources and agreement of the measured rate coefficients for the reaction of the absorbing species with O(2) and O(3) with literature values for the CH(3)CO + O(2) + M and CH(3)CO + O(3) reactions. The CH(3)CO absorption spectrum between 490 and 660 nm has a broad peak centered near 535 nm and shows no discernible structure. The absorption cross section of CH(3)CO at 532 nm was measured to be (1.1 +/- 0.2) x 10(-19) cm(2) molecule(-1) (base e).     

Reaction of HO with hydroxyacetone (HOCH2C(O)CH3): rate coefficients (233-363 K) and mechanism

Absolute rate coefficients for the title reaction, HO + HOCH(2)C(O)CH(3)--> products (R1) were measured over the temperature range 233-363 K using the technique of pulsed laser photolytic generation of the HO radical coupled to detection by pulsed laser induced fluorescence. The rate coefficient displays a slight negative temperature dependence, which is described by: k(1)(233-363 K) = (2.15 +/- 0.30) x 10(-12) exp{(305 +/- 10)/T} cm(3) molecule(-1) s(-1), with a value of (5.95 +/- 0.50) x 10(-12) cm(3) molecule(-1) s(-1) at room temperature. The effects of the hydroxy-substituent and hydrogen bonding on the rate coefficient are discussed based on theoretical calculations. The present results, which extend the database on the title reaction to a range of temperatures, indicate that R1 is the dominant loss process for hydroxyacetone throughout the troposphere, resulting in formation of methylglyoxal at all atmospheric temperatures. As part of this work, the rate coefficient for reaction of O((3)P) with HOCH(2)C(O)CH(3) (R4) was measured at 358 K: k(4)(358 K) = (6.4 +/- 1.0) x 10(-14) cm(3) molecule(-1) s(-1) and the absorption cross section of HOCH(2)C(O)CH(3) at 184.9 nm was determined to be (5.4 +/- 0.1) x 10(-18) cm(2) molecule(-1).     

Optical transitions of Er3+/Yb3+ codoped TeO2-WO3-Bi2O3 glass

Optical absorption and emission properties of the Er3+/Yb3+ codoped TeO2-WO3-Bi2O3 (TWB) glass has been investigated. The transition probabilities, excited state lifetimes, and the branching ratios have been predicted for Er3+ based on the Judd-Ofelt theory. The broad 1.5 microm fluorescence was observed under 970 nm excitation, and its full width at half maximum (FWHM) is 77 nm. The emission cross-section is calculated using the McCumber theory, and the peak emission cross-section is 1.03 x 10(-21) cm2 at 1.531 microm. This value is much larger than those of the silicate and phosphate glasses. Efficient green and weak red upconversion luminescence from Er3+ centers in the glass sample was observed at room temperature, and the upconversion excitation processes have been analyzed.     

High-resolution spectra of bromo methane and methyl bromide obtained by a continuously tunable 10-bar CO2 laser based photoacoustic spectrometer.

High resolution spectral data of 100 ppmV (10(-6) per volume) concentrations of the trace gases bromo methane (BM, CH3Br), and methyl bromide (DME, (CH3)2O), buffered in synthetic air (80% N2, 20% O2) at atmospheric pressure and room temperature are reported. The spectra are recorded with a continuously tunable 10-bar CO2 laser based photoacoustic (PA) spectrometer. The tuning range covers 76 cm(-1) between 9.2 microm (1087 cm(-1)) and 10.7 microm (935 cm(-1)) at a constant narrow line-width of 0.018 cm(-1) (540 MHz). The non-resonant PA measuring cell employs an in-line 10-microphone array. The estimated detection limits for BM and DME are approximately 2 ppmV for a signal-to-noise ratio (SNR) of 3. This corresponds to a calculated detection limit of approximately 76 ppbV for ethylene.     

The heterogeneous chemical kinetics of NO3 on atmospheric mineral dust surrogates

Uptake experiments of NO3 on mineral dust powder were carried out under continuous molecular flow conditions at 298 +/- 2 K using the thermal decomposition of N2O5 as NO3 source. In situ laser detection using resonance enhanced multiphoton ionization (REMPI) to specifically detect NO2 and NO in the presence of N2O5, NO3 and HNO3 was employed in addition to beam-sampling mass spectrometry. At [NO3] = (7.0 +/- 1.0) x 10(11) cm(-3) we found a steady state uptake coefficient gamma(ss) ranging from (3.4 +/- 1.6) x 10(-2) for natural limestone to (0.12 +/- 0.08) for Saharan Dust with gamma(ss) decreasing as [NO3] increased. NO3 adsorbed on mineral dust leads to uptake of NO2 in an Eley-Rideal mechanism that usually is not taken up in the absence of NO3. The disappearance of NO3 was in part accompanied by the formation of N2O5 and HNO3 in the presence of NO2. NO3 uptake performed on small amounts of Kaolinite and CaCO3 leads to formation of some N2O5 according to NO((3ads)) + NO(2(g)) --> N2O(5(ads)) --> N2O(5(g)). Slow formation of gas phase HNO3 on Kaolinite, CaCO3, Arizona Test Dust and natural limestone has also been observed and is clearly related to the presence of adsorbed water involved in the heterogeneous hydrolysis of N2O(5(ads)).     

N3-dye-induced visible laser anatase-to-rutile phase transition on mesoporous TiO2 films

Titanium dioxide has been extensively used in photocatalysis and dye-sensitized solar cells, where control of the anatase-to-rutile phase transformation may allow the realization of more efficient devices exploiting the synergic effects at anatase/rutile interfaces. Thus, a systematic study showing the proof of concept of a dye-induced morphological transition and an anatase-to-rutile transition based on visible laser (532 nm) and nano/micro patterning of mesoporous anatase (Degussa P25 TiO(2)) films is described for the first time using a confocal Raman microscope. At low laser intensities, only the bleaching of the adsorbed N3 dye was observed. However, high enough temperatures to promote melting/densification processes and create a deep hole at the focus and an extensive phase transformation in the surrounding material were achieved using 1s laser pulses of 25-41 mW/cm(2), in resonance with the MLCT band. The dye was shown to play a key role, being responsible for the absorption and efficient conversion of the laser light into heat. As a matter of fact, the dye is photothermally decomposed to amorphous carbon or to gaseous species (CO(x), NO(x), and H(2)O) under a N(2) or O(2) atmosphere, respectively.     

A kinetic study of the reactions of Fe+ with N2O, N2, O2, CO2 and H2O, and the ligand-switching reactions Fe+.X + Y --> Fe+.Y + X (X = N2, O2, CO2; Y = O2, H2O)

A series of reactions involving Fe(+) ions were studied by the pulsed laser ablation of an iron target, with detection of ions by quadrupole mass spectrometry at the downstream end of a fast flow tube. The reactions of Fe(+) with N(2)O, N(2) and O(2) were studied in order to benchmark this new technique. Extending measurements of the rate coefficient for Fe(+) + N(2)O from 773 K to 185 K shows that the reaction exhibits marked non-Arrhenius behaviour, which appears to be explained by excitation of the N(2)O bending vibrational modes. The recombination of Fe(+) with CO(2) and H(2)O in He was then studied over a range of pressure and temperature. The data were fitted by RRKM theory combined with ab initio quantum calculations on Fe(+).CO(2) and Fe(+).H(2)O, yielding the following results (120-400 K and 0-10(3) Torr). For Fe(+) + CO(2): k(rec,0) = 1.0 x 10(-29) (T/300 K)(-2.31) cm(6) molecule(-2) s(-1); k(rec,infinity) = 8.1 x 10(-10) cm(3) molecule(-1) s(-1). For Fe(+) + H(2)O: k(rec,0) = 5.3 x 10(-29) (T/300 K)(-2.02) cm(6) molecule(-2) s(-1); k(rec,infinity) = 2.1 x 10(-9) (T/300 K)(-0.41) cm(3) molecule(-1) s(-1). The uncertainty in these rate coefficients is determined using a Monte Carlo procedure. A series of exothermic ligand-switching reactions were also studied at 294 K: k(Fe(+).N(2) + O(2)) = (3.17 +/- 0.41) x 10(-10), k(Fe(+).CO(2) + O(2)) = (2.16 +/- 0.35) x 10(-10), k(Fe(+).N(2) + H(2)O) = (1.25 +/- 0.14) x 10(-9) and k(Fe(+).O(2) + H(2)O) = (8.79 +/- 1.30) x 10(-10) cm(3) molecule(-1) s(-1), which are all between 36 and 52% of their theoretical upper limits calculated from long-range capture theory. Finally, the role of these reactions in the chemistry of meteor-ablated iron in the upper atmosphere is discussed. The removal rates of Fe(+) by N(2), O(2), CO(2) and H(2)O at 90 km altitude are approximately 0.1, 0.07, 3 x 10(-4) and 1 x 10(-6) s(-1), respectively. The initially formed Fe(+).N(2) and Fe(+).O(2) are converted into the H(2)O complex at approximately 0.05 s(-1). Fe(+).H(2)O should therefore be the most abundant single-ligand Fe(+) complex in the mesosphere below 90 km.     

Vibrational relaxation of O2(X3 Sigma g-, v = 9-13) by collisions with O2

Vibrationally excited O(2)(X(3) Sigmag(-)) was generated in the UV laser flash photolysis of O(3) and single vibrational level was detected via laser-induced fluorescence (LIF) in the B(3) Sigmau(-)-X(3) Sigmag(-) system. The time-resolved LIF of adjacent vibrational levels has been analyzed by the integrated-profiles method and the rate coefficients for single-quantum relaxation, O(2)(X(3)Sigmag(-), v = 9-13)+ O(2)(v = 0)--> O(2)(X(3)Sigmag(-), v - 1)+ O(2)(v = 1), have been determined. To the best of our knowledge, the rate coefficients for v = 12 and 13 are measured for the first time in the present study. The efficiency of relaxation is higher at lower vibrational levels, indicating that a small energy mismatch is suitable for the energy transfer. The vibrational level dependence of all the rate coefficients for the relaxation measured in the present study and previously reported by several groups can be rationalized by the energy gap law.     

Rate coefficients for reaction and for rotational energy transfer in collisions between CN in selected rotational levels (X 2Sigma+, v=2, N=0, 1, 6, 10, 15, and 20) and C2H2

Rate coefficients (ktot,Ni) are reported (a) for total removal (reactive+inelastic) of CN(X2Sigma+,v=2,Ni) radicals from selected rotational levels (Ni=0, 1, 6, 10, 15, and 20) and (b) for state-to-state rotational energy transfer (ki-->f) between levels Ni and other rotational levels Nf in collisions with C2H2. CN radicals were generated by pulsed laser photolysis of NCNO at 573 nm. A fraction of the radicals was then promoted to a selected rotational level in v=2 using a tunable infrared "pump" laser operating at approximately 2.45 microm, and the subsequent fate of this subset of radicals was monitored using pulsed laser-induced fluorescence (PLIF). Values of ktot,Ni were determined by observing the decay of the PLIF signals as the delay between pump and probe laser pulses was systematically varied. In a second series of experiments, double resonance spectra were recorded at a short delay between the pump and probe laser pulses. Analysis of these spectra yielded state-to-state rate coefficients for rotational energy transfer, ki-->f. The difference between the sum of these rate coefficients, Sigmafki-->f, and the value of ktot,Ni for the same level Ni is attributed to the occurrence of chemical reaction, yielding values of the rotationally selected rate coefficients (kreac,Ni) for reaction of CN from specified rotational levels. These rate coefficients decrease from (7.9+/-2.2)x10(-10) cm3molecule-1 s-1 for Ni=0 to (0.8+/-1.3)x10(-10) cm3 molecule-1 s-1 for Ni=20. The results are briefly discussed in the context of microcanonical transition state theory and the statistical adiabatic channel model.