Rate Constants For Collisional Removal Research Articles

Energy transfer in the ground state of OH: Measurements of OH(υ=8,10,11) removal

OH collisional removal rate constants for υ=8, 10, and 11 with O2, N2, CO2, N2O, and He are measured. Vibrationally excited OH molecules in υ=5, 7, and 8 are prepared from the reaction of atomic hydrogen with ozone, and selectively excited to υ=8, 10, and 11, respectively, with a pulsed infrared laser via direct overtone excitation. Subsequent collisional deactivation of these laser-populated levels is probed as a function of collider gas partial pressure using a time-delayed, pulsed ultraviolet laser resonant with OH(B 2Σ+−X 2Π) transitions. Fluorescence from OH(B 2Σ+−A 2Σ+) is detected. These results complete a series of removal rate constant measurements for OH(X 2Π) up to υ=12. Resonance enhancement effects in vibration-to-vibration energy transfer for CO2 and N2O factor significantly in the vibrational-level-dependent rate constants.

Collisional removal of O2 (c 1Σ−u, ν=9) by O2, N2, and He

The collisional removal of O2 molecules in selected vibrational levels of the c 1Σ−u state is studied using a two-laser double-resonance technique. The output of the first laser excites the O2 to ν=9 or 10 of the c 1Σ−u state, and the ultraviolet output of the second laser monitors specific rovibrational levels via resonance-enhanced ionization. The temporal evolution of the c 1Σ−u state vibrational level is observed by scanning the time delay between the two pulsed lasers. Collisional removal rate constants for c 1Σ−u, ν=9 colliding with O2, N2, and He are (5.2±0.6)×10−12, (3.2±0.4)×10−12, and (7.5±0.9)×10−12 cm3 s−1, respectively. As the rate constants for O2 and N2 are similar in magnitude, N2 collisions dominate the removal rate in the earth’s atmosphere. For ν=10 colliding with O2, we find a removal rate constant that is 2–5 times that for ν=9 and that single quantum collision cascade is an important pathway for removal.

Collisional removal of OH (X 2Π,ν=7) by O2, N2, CO2, and N2O

Collisional removal rate constants for the OH (X 2Π,ν=7) radical are measured for the colliders O2, CO2, and N2O, and an upper limit is established for N2. OH(ν=4) molecules, generated in a microwave discharge flow cell by the reaction of hydrogen atoms with ozone, are excited to ν=7 by the output of a pulsed infrared laser via direct vibrational overtone excitation. The temporal evolution of the ν=7 population is probed as a function of the collider gas partial pressure by a time-delayed pulsed ultraviolet laser. The probe laser light is resonant with the (0,7) band of the B 2Σ+−X 2Π transition. Fluorescence from the B 2Σ+ state is detected in the visible spectral region. We measure rate constants for CO2, (6.7±1.0)×10−11; N2O, (3.0±0.6)×10−11; O2, (7±2)×10−12; and N2, <6×10−13 (all in units of cm3 s−1).

Laser double-resonance study of the collisional removal of O2 (A 3Σ+u, ν=6, 7, and 9) with O2, N2, CO2, Ar, and He

The collisional removal of O2 molecules prepared in selected vibrational levels of the A 3Σ+u state is studied using a two-laser double-resonance technique. The output of the first laser excites the O2 to A 3Σ+u, ν=6, 7, or 9, and the ultraviolet output of the second laser monitors these levels via resonance-enhanced ionization through either the ν=5 level of the C 3Πg Rydberg state, or the valence state or states tentatively associated with the 5 3Πg state. The temporal evolution of the A 3Σ+u state vibrational level is observed by scanning the time delay between the two pulsed lasers. Collisional removal rate constants are obtained for A 3Σ+u, ν=7 and 9 colliding with O2, N2, CO2, Ar, and He; and for ν=6 colliding with O2 and N2. We find the collisional removal of the A 3Σ+u state to be fast (k≥10−11 cm3 s−1) for all colliders studied. The rate constants vary by about an order of magnitude from the fastest collisional deactivator, CO2 to the slowest studied, the rare gases Ar and He. The rate constants for the atmospherically important colliders O2 and N2 are similar in magnitude and suggest that N2 collisions will dominate the removal rate in the Earth’s atmosphere.

Collision dynamics of OH(X 2Π, v=9)

Collisional removal rate constants for the OH(X 2Π, v=9) radical are measured for the colliders O2, CO2, and N2O and upper limits are established for He, H2, Ar, and N2. OH(v=6) molecules, generated in a microwave discharge flow cell by the reaction of hydrogen atoms with ozone, are excited to v=9 by the output of a pulsed infrared laser via direct vibrational overtone excitation. The temporal evolution of the v=9 population is probed as a function of the collider gas partial pressure by a time-delayed pulsed ultraviolet laser. The probe laser light is resonant with the B 2Σ+–X 2Π(0,9) transition and the resulting visible B 2Σ+–A 2Σ+ fluorescence is detected with a filtered photomultiplier tube. We measure rate constants for N2O: (6.4±1.0)×10−11; CO2: (5.7±0.6)×10−11; O2: (1.7±1.1)×10−11; H2: <3×10−12; He: <2×10−12; N2: <5×10−13; Ar: <2×10−13 (all in units of cm3 s−1). For O2 and CO2 these rate constants are significantly faster than those for low vibrational levels and comparable to those for v=12.

State-to-state rotational energy transfer and reaction with ketene of highly vibrationally excited b̃ 1B1 CH2 by time-resolved Fourier transform emission spectroscopy

Dispersed fluorescence spectra from the CH2 b̃ 1B1→ã 1A1 band were recorded with time-resolution by Fourier transform emission spectroscopy after pulsed excitation of a single rotational level of the b̃ 1B1 (0,160,0) state. Fluorescence observed from the initially excited level and from levels populated by rotational energy changing collisions with the bath gas (ketene) was used to deduce the state-to-state rate constants for rotational energy transfer and the state-resolved rate constants for total collisional removal of b̃ 1B1 CH2. The observed propensity rules for rotational energy transfer—ΔJ=±2, ΔKa=0, and ΔKc=±2—are consistent with a quadrupole–dipole interaction between b̃ 1B1 (0,160,0) CH2 and ketene. The existence of a quadrupole in the intermolecular interaction suggests that the structure of CH2 in the b̃ 1B1 (0,160,0) state, averaged over the time of a collision, must be linear. The state-to-state rotational energy transfer rate constants range from approximately equal to the hard sphere gas kinetic rate to four times the gas kinetic rate, with the largest rate constants between rotational levels with the smallest energy gaps. Examination of fluorescence spectra recorded with polarization analysis shows that rotationally elastic (ΔJ=0)M changing collisions are negligible. State-resolved rate constants for reactive collisions between b̃ 1B1 CH2 and ketene were obtained by subtracting the rotational energy transfer contribution from the total rate constants for collisional removal of b̃ 1B1 CH2 (obtained from a Stern–Volmer analysis). These rate constants vary from one to five times the hard sphere gas kinetic rate, and increase with rotational energy for the levels studied. Their magnitudes show that CH2 is about two times as reactive in its b̃ 1B1 state than its ã 1A1 state.

Direct Measurements of Removal Rates of CHF(X˜1A′(0,1,0)) by Simple Alkenes

Direct measurements of the rate constants for collisional removal of CHF(X˜1A′(0,1,0)) by argon (Ar), ethylene (C2H4), propene (C3H6), 1-butene (1-C4H8), iso-butene (i-C4H8), 1,3-butadiene (C4H6), are reported. CHF(X˜1A′(0,1,0)) was prepared by IRMPD of precursor CH2F2. Fluence independence removal rate constants by Ar, ethylene and propene are: 0.0045, 2.70 and 2.46 × 10−11 cm3 molecule−1 s−1, faster than the rates for the vibrational ground state. 1,3-butadiene was measured at a fluence of 42Jcm−2, yielding a value of 0.81 × 10−11 cm3 molecule−1 s−1. Iso-butene has fluence dependence removal rates, given 1.28 and 0.92 × 10−11 cm3 molecule−1 s−1 for fluences of 50 and 32Jcm−2 respectively. For 1-butene a non-linear Stern-Volmer behaviour has been observed and attributed to fast resonant vibrational-to-vibrational (V-to-V) intermolecular energy transfer. The influence of this (V-to-V) energy transfer on the removal rates is discussed.

Collision dynamics of OH(X 2Πi,v=12)

Collisional removal rate constants for the OH radical in v=12 of the ground electronic state are measured for the colliders CO2, O2, N2, H2, He, and Ar. OH molecules, generated in v=8 by the reaction of hydrogen atoms with ozone, are excited to v=12 by direct overtone excitation with pulsed infrared laser light. The temporal evolution of the v=12 radicals is probed as a function of collider gas pressure by a time-delayed pulsed ultraviolet probe laser. The probe laser is used to excite the molecules via the B 2Σ+–X 2Πi(0,12) electronic transition, and the resulting B 2Σ+–A 2Σ+ fluorescence is detected. We measure rate constants for CO2:(5.6±1.5)×10−11; O2:(1.6±0.2)×10−11; He:(3.6±0.6)×10−12; H2:(3.0±0.8)×10−12; Ar:(2.6±0.5)×10−12; N2:(2.5±0.7)×10−12 (all in units of cm3 s−1). These rate constants are over fifty times faster in all cases than the vibrational relaxation rate constants for the lower levels (v=1 and v=2) of the ground state.

Time-resolved CH (A^2Δ and B^2∑^−) laser-induced fluorescence in low pressure hydrocarbon flames

Total collisional removal rate constants for the CH A(2)Delta and B(2)Sigma(-) electronic states are obtained in low pressure (<20-Torr) hydrocarbon flames. The B state is consistently removed ~70% faster than the A state. Variations of +/-50% are observed for different rotational levels and positions in the flame. For these flames, A-state emission following excitation of the B state indicates a rapid electronic-to-electronic energy transfer pathway that is insensitive to collision environment. Upper limits to the collider specific cross sections are obtained for H(2)O, N(2), and CO(2). The CH concentration and temperature profiles are measured and parametrized using a unique method.

Collisional quenching and energy transfer in NS B 2Π

Total collisional removal rate constants kd for the B 2Π excited electronic state of the NS free radical are measured for several collider molecules. For the lowest vibrational level (v′=0), kd is the electronic quenching rate constant; and for the vibrationally excited v′=1 and v′=6 levels, kd is the sum of those for electronic quenching and vibrational relaxation. Nitrogen sulfide free radicals are produced in a discharge flow reactor and the B 2Π state is prepared and monitored by laser-induced fluorescence. Measurements are made for nine different collision partners: He, N2, O2, SF6, N2O, H2, CH4, CO2, and NH3. Except for NH3, the thermally averaged quenching cross sections at room temperature are less than 10 Å2; this is a small value for electronic quenching of open shell diatomic radicals. For v′=6, kd is smaller than for v′=1 for N2, SF6, N2O, and CO2, but larger for H2, O2, and He. The vibrational relaxation pathway from v′=1 to v′=0 constitutes about 25% of the total v′=1 collisional removal for O2, N2O, and SF6.

Infra-red multiple-photon dissociation as a source of reactive free radicals: Methylene revisited

Rate constants for collisional removal of ã1 A 1 CH2 and CD2 have been directly measured for the first time, by using IR multiplephoton dissociation to produce the radicals and time-resolved laser-induced fluorescence to detect them. Contributions to the removal processes from both intersystem crossing and chemical reaction are discussed.

Singlet methylene kinetics: Direct measurements of removal rates of ã 1A 1 and b̃ 1B 1 CH 2 and CD 2

Rate constants for collisional removal of ã 1A 1 and b̃ 1B 1 CH 2 and CD 2 have been directly measured, using IR laser induced multiple photon dissociation to prepare the radicals, and time resolved laser induced fluorescence to observe them. For CH 2(ã 1A 1) removal by He, Ne, Ar, Kr, Xe, N 2, H 2, O 2, CO and CH 4, rate constants of 3.1, 4.2, 6.0, 7.0, 16, 8.8, 130, 30, 56 and 73 × 10 −12 cm 3 molecule −1 s −1 were found respectively. These represent significant increases over the previously accepted values. Essentially no isotope effect is observed in the removal of CD 2(ã 1A 1) by the rare gases. The rate determining step in removal by the rare gases and N 2 is thought to be singlet—triplet intersystem crossing controlled by long range attractive forces, and the results are discussed in terms of both isolated and mixed state theoretical models of these processes. For the other molecular collision partners, bimolecular chemical removal channels are possible, and may account for the relatively fast rates observed. Radiative lifetimes of five Σ vibronic levels of CH 2(b̃ 1B 1) and three Σ vibronic levels of CD 2(b̃ 1B 1) have been measured and found to lie in the range 2.5–6.0 μs, and collisional quenching rates for CH 2(b̃ 1B 1) are found to be of the order of the gas kinetic collisional frequency.