The 193 nm photolysis of NO2: NO(ν) vibrational distribution, O(1D) quantum yield and emission from vibrationally excited NO2

Slice imaging study of NO2 photodissociation via the 12B2 and 22B2 states: the NO(X2Π) + O(3PJ) product channel

The state-resolved photodissociation of NO₂via the 1²B₂ and 2²B₂ excited states has been investigated by using time-sliced velocity-mapped ion imaging technique. The images of the O(³P_J=2,1,0) products at a series of excitation wavelengths are measured by employing a 1 + 1' photoionization scheme. The total kinetic energy release (TKER) spectra, NO vibrational state distributions and anisotropy parameters (β) are derived from the O(³P_J=2,1,0) images. For the 1²B₂ state photodissociation of NO₂, the TKER spectra mainly present a non-statistical vibrational state distribution of the NO co-products, and the profiles of most vibrational peaks display a bimodal structure. The β values show a gradual decrease with the photolysis wavelength increasing except for a sudden increase at 357.38 nm. The results suggest that the NO₂ photodissociation via the 1²B₂ state proceeds via the non-adiabatic transition between the 1²B₂ and X̃²A₁ states, leading to the NO(X²Π) + O(³P_J) products with wavelength-dependent rovibrational distributions. As for photodissociation of NO₂via the 2²B₂ state, the NO vibrational state distribution is relatively narrow with the main peak shifting from v = 1, 2 at 235.43-249.22 nm to v = 6 at 212.56 nm. The β values exhibit two distinctly different angular distributions, i.e., near isotropic at 249.22 and 246.09 nm and anisotropic at the rest of the excitation wavelengths. These results are consistent with the fact that the 2²B₂ state potential energy surface has a barrier, and the dissociation process is fast when the initial populated level is above this barrier. A bimodal vibrational state distribution is clearly observed at 212.56 nm, in which the main distribution (peaking at v = 6) is ascribed to dissociation via an avoided crossing with the higher electronically excited state while the subsidiary distribution (peaking at v = 11) likely arises due to dissociation via the internal conversion to the 1₂B₂ state or to the X̃ ground state.

Time-resolved observations of vibrationally excited NO X 2Π (v') formed from collisional quenching of NO A 2Σ+ (v = 0) by NO X 2Π: evidence for the participation of the NO a 4Π state

Time-resolved observations have been made of the formation of vibrationally excited NO X ²Π (v') following collisional quenching of NO A ²Σ⁺ (v = 0) by NO X ²Π (v = 0). Two time scales are observed, namely a fast production rate consistent with direct formation from the quenching of the electronically excited NO A state, together with a slow component, the magnitude and rate of formation of which depend upon NO pressure. A reservoir state formed by quenching of NO A ²Σ⁺ (v = 0) is invoked to explain the observations, and the available evidence points to this state being the first electronically excited state of NO, a ⁴Π. The rate constant for quenching of the a ⁴Π state to levels v' = 11-16 by NO is measured as (8.80 ± 1.1) × 10^-11 cm³ molecule^-1 s^-1 at 298 K where the error quoted is two standard deviations, and from measurements of the increased formation of high vibrational levels of NO(X) by the slow process we estimate a lower limit for the fraction of self-quenching collisions of NO A ²Σ⁺ (v = 0) which lead to NO a ⁴Π as 19%.

Characterization of two photon excited fragment spectroscopy (TPEFS) for HNO3 detection in gas-phase kinetic experiments

We have developed and tested two-photon excited fragment spectroscopy (TPEFS) for detecting HNO₃ in pulsed laser photolysis kinetic experiments. Dispersed (220-330 nm) and time-dependent emission at (310 ± 5) nm following the 193 nm excitation of HNO₃ in N₂, air and He was recorded and analysed to characterise the OH(A²Σ) and NO(A²Σ⁺) electronic excited states involved. The limit of detection for HNO₃ using TPEFS was ∼5 × 10⁹ molecule cm^-3 (at 60 torr N₂ and 180 μs integration time). Detection of HNO₃ using the emission at (310 ± 5 nm) was orders of magnitude more sensitive than detection of NO and NO₂, especially in the presence of O₂ which quenches NO(A²Σ⁺) more efficiently than OH(A²Σ). While H₂O₂ (and possibly HO₂) could also be detected by 193 nm TPEFS, the relative sensitivity (compared to HNO₃) was very low. The viability of real-time TPEFS detection of HNO₃ using emission at (310 ± 5) nm was demonstrated by monitoring HNO₃ formation in the reaction of OH + NO₂ and deriving the rate coefficient, k₂. The value of k₂ obtained at 293 K and pressures of 50-200 torr is entirely consistent with that obtained by simultaneously measuring the OH decay and is in very good agreement with the most recent literature values.

Collisional Energy Transfer from Vibrationally Excited Hydrogen Isocyanide

Collisional deactivation of vibrationally excited hydrogen isocyanide (HNC) by inert gas atoms was characterized using nanosecond time-resolved Fourier transform infrared emission spectroscopy. HNC, with an average nascent internal energy of 25.9 ± 1.4 kcal mol^-1, was generated following the 193 nm photolysis of vinyl cyanide (CH₂CHCN) and collisionally deactivated with the series of inert atomic gases: He, Ar, Kr, and Xe. Time-dependent IR emission allows simultaneous experimental observation of the ν₁ NH and ν₃ NC stretch emissions from vibrationally excited HNC. Subsequent spectral fit analysis enables direct determination of the average energy of HNC in each spectrum and therefore a measure of the average energy lost per collision, ⟨ΔE⟩, as a function of internal energy. Collisional deactivation of excited HNC is shown to be relatively efficient, exhibiting ⟨ΔE⟩ values more than an order of magnitude larger than comparably sized molecules at similar internal energies. Furthermore, the lighter inert gases are shown to be more efficient quenchers. Both observations can be qualitatively explained by the momentum gap law modeled through the repulsive force dominated vibration-to-translation energy transfer mechanism. The feasibility of efficient collisional deactivation as a contributing factor to the observed overabundance of astrophysical HNC is discussed.

An FTIR emission study of the products of NO A2Σ+ (v = 0, 1) + O2 collisions

Collisional quenching of NO A²Σ⁺ (v = 0, 1) by O₂ has been studied through the detection of vibrationally excited products by time-resolved Fourier transform infrared emission spectroscopy. Non-reactive quenching of NO A²Σ⁺ (v = 0) produces a vibrational distribution in NO X²Π which has been quantified for v = 2-22, and is found to be bimodal. The results are consistent with two quenching channels. The first forms the ground X³Σ or low-lying a ¹Δ_g electronic state of O₂ with a distribution including high vibrational levels of NO X²Π which is slightly hotter than statistical. Two possibilities are identified for the second channel. The first, with a similar quantum yield to that producing higher vibrational levels, forms a highly electronically excited state, such as O₂ c¹Σ, with low vibrational levels in NO X²Π which are inverted with a distribution resembling that resulting from a sudden or harpoon mechanism. The second is that ground state oxygen is formed with low vibrational energy partitioned into NO X²Π. In addition, vibrationally excited NO₂ is observed, but at intensities which indicate that it is formed in low quantum yield. Quantitatively unobservable processes (defined as those which do not form ground state NO (v ≥ 2)) are found to have a branching ratio of at most 25 ± 5%. The results are compared with those of previous studies and the most consistent interpretation suggests that dissociation of O₂ to form ground state O(³P) atoms and ground vibrational state NO X²Π (v = 0) is the main reactive process rather than NO₂ formation. Qualitatively similar results are seen for the quenching of NO A²Σ⁺ (v = 1).

Quantum interference in NO2

This paper investigates the origin of a quantum interference observed when NO(2) is dissociatively ionized by short pulses of ultraviolet light. We describe time-resolved measurements of NO(+), O(+), and NO(2)(+) ions produced following the interaction of NO(2) with a approximately 70 fs duration pulse centered close to 400 nm and a subsequent time-delayed probe pulse close to 269, 205, or 400 nm. A quantum beat oscillation with a period of 524 fs and a characteristic damping time of 8 ps is observed on all transient ion signals. We investigate the effect of tuning the central wavelength of the excitation pulse over a 12 nm range, and we discuss the potential importance of three possible multiphoton pathways involving one, two, and three pump photons. We conclude that the ionization pathway responsible for the beat signal is most likely due to a process involving the absorption of two pump photons and two probe photons. This presents an interesting problem with respect to the interpretation of the mechanism responsible for the quantum interference signature since the electronic states of NO(2) reached at the two-photon level are all thought to be extremely short-lived and to dissociate on a time scale that is far shorter than the characteristic damping time of the oscillatory signals. We suggest that a possible explanation for the observed dynamics is associated with a minor dissociation channel of the (2)(2)B(2) state of NO(2) through its interaction with the longer lived (2)(2)A(1) state.

Vibrational relaxation of NO (v = 1-16) with NO, N2O, NO2, He and Ar studied by time-resolved Fourier transform infrared emission

Rates of vibrational quenching of NO (v = 1-16) in collisions with a series of quenching species NO, NO2, N2O, He and Ar have been measured at 295 K. NO (v) was formed both by the O(1D) + N2O reaction and the 193 nm photolysis of NO(2), and time-resolved FTIR emission was used to follow the behaviour of the vibrationally excited species. The trends in quenching rate constants can be explained in terms of V-T transfer, V-V transfer and by the effects of competing processes. He and Ar show trends expected from SSH theory, but with relaxation rates that are considerably higher than those expected from previous studies with closed shell molecules, and the influence of non-adiabatic pathways in the relaxation of the NO 2Pi state is discussed. Relaxation with NO2 shows the influence of resonant energy transfer to the nu3 mode, with rate constants peaking at v = 10. For N2O, relaxation rates show essentially a linear increase with v. A linear increase is expected for the change of the transition moment with v for the harmonic oscillator approximation, and when this is taken into account the "reduced probabilities" (defined as P/v, where P is probability of a gas kinetic collision changing the vibrational level from v to v-1) are approximately independent of the energy lost in the NO molecule. The influence of complex formation far from resonance is invoked in both this and for quenching of low vibrational levels by NO2. Finally, self-quenching shows rates which initially decrease with increasing v, but then show a marked increase, with a minimum value at v = 9. Both V-V and V-T processes are believed to occur. Where previously published data are available, general agreement is observed in this study.

Photodissociation of NO2 in the (2) (2)B2 state: the O((1)D2) dissociation channel

Direct current slice and crush velocity map imaging has been used to probe the photodissociation dynamics of nitrogen dioxide above the second dissociation limit. The paper is a companion to a previous publication [J. Chem. Phys. 128, 164318 (2008)] in which we reported results for the O((3)P(J)) + NO((2)Pi(Omega)) adiabatic product channel. Here we examine the O((1)D(2)) + NO((2)Pi(Omega)) diabatic product channel at similar excitation energies. Using one- and two-color imaging experiments to observe the velocity distributions of state selected NO fragments and O atoms, respectively, we are able to build a detailed picture of the dissociation dynamics. We show that by combining the information obtained from velocity map imaging studies with mass-resolved resonantly enhanced multiphoton ionization spectroscopy it is possible to interpret and fully assign the NO images. By recording two-color images of the O((1)D(2)) photofragments with different polarization combinations of the pump and probe laser fields we also measure the orbital angular momentum alignment in the atomic fragment. We find that the entire O((1)D(2)) photofragment distribution is similarly aligned with most of the population in the M(J) = +/-1 magnetic sublevels. The similarity of the fragment polarizations is interpreted as a signature of all of the O((1)D(2)) atoms being formed via the same avoided crossing. At the photolysis energy of 5.479 52 eV we find that the NO fragments are preferentially formed in v = 1 and that the vibrationally excited fragments exhibit a bimodal rotational distribution. This is in contrast to the unimodal rotational profile of the NO fragments in v = 0. We discuss these observations in terms of the calculated topology of the adiabatic potential energy surfaces and attribute the vibrational inversion and rotational bimodality of the v = 1 fragments to the symmetric stretch and bending motion generated on excitation to the (2) (2)B(2) state.

Photochemical removal of NO(2) by using 172-nm Xe(2) excimer lamp in N(2) or air at atmospheric pressure

Photochemical removal of NO(2) in N(2) or air (5-20% O(2)) mixtures was studied by using 172-nm Xe(2) excimer lamps to develop a new simple photochemical aftertreatment technique of NO(2) in air at atmospheric pressure without using any catalysts. When a high power lamp (300 mW/cm(2)) was used, the conversion of NO(2) (200-1000 ppm) to N(2) and O(2) in N(2) was >93% after 1 min irradiation, whereas that to N(2)O(5), HNO(3), N(2), and O(2) in air (10% O(2)) was 100% after 5s irradiation in a batch system. In a flow system, about 92% of NO(2) (200 ppm) in N(2) was converted to N(2) and O(2), whereas NO(2) (200-400 ppm) in air (20% O(2)) could be completely converted to N(2)O(5), HNO(3), N(2), and O(2) at a flow rate of 1l/min. It was found that NO could also be decomposed to N(2) and O(2) under 172-nm irradiation, though the removal rate is slower than that of NO(2) by a factor of 3.8. A simple model analysis assuming a consecutive reaction NO(2)-->NO-->N+O indicated that 86% of NO(2) is decomposed directly into N+O(2) and the rest is dissociated into NO+O under 172-nm irradiation. These results led us to conclude that the present technique is a new promising catalyst-free photochemical aftertreatment method of NO(2) in N(2) and air in a flow system.

Vibrational distribution in NO(X2Pi) formed by self quenching of NO A 2Sigma+ (v=0)

Time-resolved FTIR has been used to study the emission from the NO X 2Pi (v) products formed both by fluorescence and by collisional self quenching of the NO A 2Sigma+ (v=0) state. Vibrational excitation has been observed in ground state NO with populations up to at least v=20. Under conditions where fluorescence is the dominant removal process the nascent distribution in ground state NO(v) was found to be determined by the relative magnitude of the emission coefficients. Collisional quenching by ground state NO populates higher vibrational levels in NO(v) than fluorescence. By comparing distributions acquired at different pressures and by using a surprisal analysis, a nascent distribution of NO(v=0-20) is estimated for collisional relaxation of NO A 2Sigma+ (v=0) by NO. This distribution was found to be slightly hotter than statistical (prior) and showed evidence of oscillations at specific vibrational levels. This work is one of the first to be published concerning the vibrational ground state products of the quenching of electronically excited molecules and the first to report emission over such a large number of vibrational levels.

Quenching of I(2P1/2) by NO2, N2O4, and N2O

Quenching of excited iodine atoms (I(5p5, 2P1/2)) by nitrogen oxides are processes of relevance to discharge-driven oxygen iodine lasers. Rate constants at ambient and elevated temperatures (293-380 K) for quenching of I(2P1/2) atoms by NO2, N2O4, and N2O have been measured using time-resolved I(2P1/2) --> I(2P3/2) 1315 nm emission. The excited atoms were generated by pulsed laser photodissociation of CF3I at 248 nm. The rate constants for I(2P1/2) quenching by NO2 and N2O were found to be independent of temperature over the range examined with average values of (2.9 +/- 0.3) x 10(-15) and (1.4 +/- 0.1) x 10(-15) cm3 s(-1), respectively. The rate constant for quenching of I(2P1/2) by N2O4 was found to be (3.5 +/- 0.5) x 10(-13) cm3 s(-1) at ambient temperature.

The photodissociation dynamics of NO2 at 308nm and of NO2 and N2O4 at 226nm

Velocity-map ion imaging has been applied to the photodissociation of NO(2) via the first absorption band at 308 nm using (2 + 1) resonantly enhanced multiphoton ionization detection of the atomic O((3)P(J)) products. The resulting ion images have been analyzed to provide information about the speed distribution of the O((3)P(J)) products, the translational anisotropy, and the electronic angular momentum alignment. The atomic speed distributions were used to provide information about the internal quantum-state distribution in the NO coproducts. The data were found to be consistent with an inverted NO vibrational quantum-state distribution, and thereby point to a dynamical, as opposed to a statistical dissociation mechanism subsequent to photodissociation at 308 nm. Surprisingly, at this wavelength the O-atom electronic angular momentum alignment was found to be small. Probe-only ion images obtained under a variety of molecular-beam backing-pressure conditions, and corresponding to O atoms generated in the photodissociation of either the monomer, NO(2), or the dimer, N(2)O(4), at 226 nm, are also reported. For the monomer, where 226 nm corresponds to excitation into the second absorption band, the kinetic-energy release distributions are also found to indicate a strong population inversion in the NO cofragment, and are shown to be remarkably similar to those previously observed in the wavelength range of 193-248 nm. Mechanistic implications of this result are discussed. At 226 nm it has also been possible to observe directly O atoms from the photodissociation of the dimer. The O-atom velocity distribution has been analyzed to provide information about its production mechanism.

Time-resolved photoion and photoelectron imaging of NO2

Time-resolved photoion and photoelectron velocity mapped images from NO(2) excited close to its first dissociation limit [to NO(X(2)Pi) + O((3)P(2))] have been recorded in a two colour pump-probe experiment, using the frequency-doubled and frequency-tripled output of a regeneratively amplified titanium-sapphire laser. At least three processes are responsible for the observed transient signals; a negative pump-probe signal (corresponding to a 266 nm pump), a very short-lived transient close to the cross-correlation of the pump and probe pulses but on the 400 nm pump side, and a longer-lived positive pump-probe signal that exhibits a signature of wavepacket motion (oscillations). These transients have two main origins; multiphoton excitation of the Rydberg states of NO(2) by both 266 and 400 nm light, and electronic relaxation in the 1(2)B(2) state of NO(2), which leads to a quasi-dissociated NO(2) high in the 1(2)A(1) electronic ground state and just below the dissociation threshold. The wavepacket motion that we observe is ascribed to states exhibiting free rotation of the O atom about the NO moiety. These states, which are common for loosely bound systems such as a van der Waals complex but unusual for a chemically-bound molecule, have previously been observed in the frequency domain by optical double resonance spectroscopy but never before in the time domain.