Photodissociation Of NO2 Research Articles

The Doppler spectra of O(1D) from the photodissociation of O2, NO2, and N2O

The Doppler profiles of the O(1D) products from the photodissociation of O2 at 157.6 nm and NO2 and N2O at 205.47 nm are detected by a resonance enhanced multiphoton ionization technique. The translation energy and angular distributions are deduced for the O(1D) atoms. Present results indicate that (a) O2 (B 3Σ−u) has a short dissociative lifetime when irradiated at 157.6 nm, (b) NO(2Π) photofragment from NO2 at 205.47 nm is mostly vibrationally excited, and (c) most of the available energy is released as kinetic energy when N2O is photodissociated at 205.47 nm.

Growth and Properties of PbTiO3 Thin Films by Photoenhanced Chemical Vapor Deposition

The photoenhanced chemical vapor deposition (CVD) of PbTiO3 thin films was carried out using tetraethyl lead, titanium tetraisopropoxide and nitrogen dioxide. The use of NO2 and UV light irradiation during the growth run affected the crystalline quality and dielectric properties of films. The photodeposited films with a perovskite structure were obtained on sapphire at substrate temperatures above 530°C. In this study, it was found that the photodissociation of NO2 played an important role in decreasing the growth temperature. The step coverage characteristics of films were also investigated.

Product rotational alignment for the reaction O(3P)+ CS(X1Σ+)→ CO(X1Σ+)+ S(3P)

Rotational alignment of the CO(X 1Σ+, v′= 14) product of the O(3P)+ CS(X 1Σ+) reaction has been measured relative to the velocity vector k of the reagents. O(3P) atoms were produced with k aligned in the laboratory frame by pulsed laser photolysis of NO2, and the CO product was detected by polarised laser-induced fluorescence. Transformation of the measured laboratory-frame rotational alignments to the required values of the alignment parameter 〈P2(J′·k)〉 were carried out using previously determined values of the translational anisotropy for the photodissociation of NO2, making allowances for both the thermal distribution of CS radicals and the spread of recoil energies of the O-atom fragment. Values of 〈P2(J′·k)〉 were measured for J′ between 12 and 35, and found to be close to zero to within the range 0 ± 0.25, with the mean value being slightly positive. Measurements of the Doppler profiles of the transitions are in qualitative agreement with those predicted for an isotropic distribution of product velocities about the k direction. These preliminary results illustrate the scope of laser based methods of extracting quantum-state-resolved data on scattering dynamics under experimental conditions which do not involve the use of molecular beam methods.

Near threshold channel selective photodissociation of NO2

The dynamics of the photodissociation of NO2 into NO(X2ΠΩ″, ν″=0,J″)+O(3P0,1,2) is investigated near the thermodynamic threshold. Cooling the internal degrees of freedom by a supersonic beam expansion provides a nearly complete quantum state selection prior to the predissociation. Measurements of the wavelength dependent dissociation yield into specific product quantum states are reported. At certain wavelengths Λ″ doublet resolved rotational population distributions of the fragments are obtained. Up to an excess energy ofEexc=121 cm−1 about 42% of this energy is partitioned into the rotation of the NO molecules, and correspondingly 58% into the translational degree of freedom. The role of electronic and total parity is discussed.

Quantum-resolved analysis of electronically-stimulated NO2 dissociation on Pt(111)

The electronically-stimulated dissociation of NO2 on Pt(111) has been studied through state-selective, time-of-flight detection of the NO product above the surface. The NO leaves as a direct dissociation product resulting from 5-800 eV electron impact, whereas the O atom remains on the surface. The quantum-resolved analysis of the NO reveals fundamental aspects of the stimulated surface process such as dominant excitation channels and dynamics. Because of rapid decay via resonant tunneling from substrate levels, the 3 eV shallow valence excitations prominent in gas-phase photodissociation of NO2 have lifetimes that are too short to produce observable dissociation on the metal surface. Instead, we find a 10-15 eV threshold which corresponds to ionization of 3b2 and double ionization of 1a2 levels which cannot be resonantly filled by substrate electrons and thus have the longest lifetimes relative to Auger decay. Screening of the hole(s) by the metal through the 6a1 orbital of the molecule not only determines the lifetime, but is found to influence the extent of internal excitation in the NO product.

Cross sections and NO product state distributions resulting from substrate mediated photodissociation of NO2 adsorbed on Pd(111)

Ultraviolet irradiation of NO2 adsorbed on top of a NO saturated Pd(111) surface causes the photodissociation of NO2/N2O4 and results in the desorption of NO molecules. This process has been studied using excitation energies between 3.5 and 6.4 eV. At a photon energy of 6.4 eV, a cross section of 3×10−18 cm2 is found. Using laser-induced fluorescence to detect the desorbed NO molecules, fully state-resolved data detailing the energy channeling into different degrees of freedom has been obtained. Two desorption channels are found, one characterized by nonthermal state populations, and one showing accommodation to the surface. The yield of the fast channel shows a marked increase above 4 eV photon energy. The slow channel is interpreted as being due to NO molecules which, after formation, undergo a trapping–desorption process. A polarization experiment indicates that the photodissociation is initiated by excitation of metal electrons rather than direct absorption by the adsorbate.

NO(B 2Π) radiative lifetimes: v=0–6

The zero-pressure radiative lifetime of the NO(B 2Π) state has been measured over the vibrational level range v=0–6. Laser-induced fluorescence was the technique chosen for this study, using two different sources of ground state NO. In one case, photodissociation of NO2 at 193 nm was used to obtain a range of ground state vibrational levels, from which selected rotational levels were then pumped to the B 2Π state, while in the second case, NO in v=0 was directly pumped. The two methods of preparing the excited state gave identical lifetime results. The data show a linearly decreasing lifetime with increasing vibrational level and to a good approximation the lifetimes, are given by τ(μs)=2.00–0.193v. Recent calculations for the B–X system show excellent agreement with experiment at low v, and an increasing discrepancy with increasing vibrational level, the experimental lifetimes decreasing more rapidly than the calculated ones. The lifetime values fall within the 0.85–2.0 μs range for v=0–6, and are substantially different from the 2–3 μs values that are currently quoted. Comparison of the new values with data derived from absorption studies shows good agreement even for the high vibrational levels. Absolute Einstein A factors are presented for the v′=6 progression.

Role of atomic oxygen in the low-temperature growth of YBa2Cu3O7−δ thin films by laser ablation deposition

Thin films of YBa2Cu3O7−δ were deposited on (100) SrTiO3 substrates held at 600 and 700 °C in N2O and O2 ambients using 355 nm Nd-YAG laser pulses for ablation of the target. The experiments were done either in the presence or absence of 193 nm excimer laser irradiation of the ambient gas between the target and the substrate. Results without the excimer irradiation show that in 0.2 Torr of both N2O and O2, at 700 °C substrate surface temperature, excellent smooth films with Tc (R=0) of 93 K and Jc (88 K) of 1.3×106 A/cm2 were obtained. At 600 °C, semiconducting films with no superconducting transition were obtained in O2 ambient, whereas in N2O, semiconducting normal state behavior with broad superconducting transition was found. With the 193 nm irradiation, no change was observed in the electrical properties of the films deposited in O2 at 600 °C, whereas in N2O reasonably good superconducting films with normal metallic behavior and Tc (R=0) of 84 K were found. Since the 193 nm photons hardly dissociate O2 molecules, but very efficiently photodissociate the N2O molecules to form N2 and O(1D), it is concluded that the atomic oxygen produced by photodissociation of N2O is responsible for the superconducting film deposition at 600 °C.

Photodissociation of NO2. Internal energy distribution and anisotropies in the fragments

The near-UV photodissociation of NO2 has been investigated at excess energies from −600 up to 1700 cm−1 above the NO X ν = 1 + O3P dissociation limit. A complete analysis of the NO X ν = 1 fragment, including internal and translational energy as well as the angular properties of its vectorial parameters, has been carried out using REMPI combined with TOF mass spectrometry. The high-precision state-selective measurements of the NO fragment velocity have been used to evaluate the internal energy of the O atoms formed in coincidence. Concerning the internal energy of the NO X ν = 1 fragment, two types of final state distributions of the fragments have been found; the difference depending upon the excess energy with respect to the NO X ν = 1 + O3P dissociation limit. Above the threshold, statistical rotational behaviour with nonvanishing anisotropies has been observed. Below the threshold, an efficient R→V transfer, allowed by a significant molecular distortion, was detected. The influence of the parent rotation on the NO fragment rotational distribution is discussed.

Two-photon photodissociation of jet-cooled NO2 near 450 nm. Rotational distributions of NO

The two-photon photodissociation of expansion-cooled NO2 has been studied using dissociation and simultaneous two-photon l.i.f probing of the nascent NO(X) by a single scanning dye laser in the 428, 450 and 475 nm wavelength range corresponding to the γ(2, 1)γ(1, 0)γ(0, 0) and γ(0, 1) two-photon bands of NO. While the nascent NO formed in the 450 nm region exhibits a Boltzmann-like rotational population distribution, a distinct bimodal distribution is observed in the 475 nm region. Attention is drawn to the possibility that these differences reflect different branching ratios between the O(3P) and O(1D) dissociation channels and that the branching ratio varies with wavelength and the nature of the intermediate state involved at the one-photon level.

Polarized absorption spectroscopy of Λ-doublet molecules: Transition moment vs electron density distribution

Polarized two-photon photodissociation of NO2 in the region of 480 nm yields NO(v″=0) with an anisotropic distribution of J. Measured polarization ratios are compared to quantum mechanical calculations for a range of expected ratios for the various isolated and mixed branches of the NO X̃2Π1/2→Ã 2Σ+ transition. Theoretical results show that main branches and their respective satellites (e.g., R11 and R21 branches) have the same transition moment directionally, though their intensities are in general different, implying that care is needed in interpreting polarization data from the mixed branches, such as (Q21+R11) or (Q11+P21), which measure the Π+ and Π− Λ doublet, respectively. Recognition of this fact is particularly important for properly separating the consideration of electron density distributions of Λ doublets from transition moment directionalities, as this has been a source of confusion in the literature. The measured results indicate that the principal two-photon photoexcitation pathway in NO2 photolysis is 2A1→1 2B2→2 2B2, with moderate A″ state mixing in the intermediate.

Primary quantum yields of NO2 photodissociation

A large discrepancy exists among recent estimates of the primary quantum efficiencies (ϕ1) of nitrogen dioxide photodecomposition, NO2 + hν → NO + O(3P), in the wavelength range from 374 to 396 nm. To resolve this problem, quantum yields of formation of NO, O2, and NO2 loss have been measured for NO2 vapor at low pressures (0.13–0.30 torr) irradiated at selected wavelengths (334.1–404.3 nm) and temperatures (273–370 K). From these data, estimates of ϕ1 were derived which confirm the previous findings of Jones and Bayes (1973b): ϕ1 increases rapidly from near zero at 424 nm to near unity for excitation at λ < 394 nm. The temperature and wavelength dependences of ϕ1 appear to be in qualitative accord with the simple theory of Pitts et al. (1964) that the energy deficiency for photodissociation of NO2 excited at λ > 397.9 nm (λdiss) is made up in large part from the rotational and vibrational energy of the NO2 molecules. Recommended values for ϕ1 based upon a review of these data and estimates made over a period of 58 years, are given as a function of wavelength.

Angular distributions of photofragments from NO2 photodissociation

The angular distributions of recoil of NO molecules from the photodissociation of NO2 near the dissociation threshold of 397.86 nm are presented and analyzed to obtain information about the effect of molecular rotation of parent molecules on the angular distribution. For the one-photon dissociation process, as laser wavelengths become closer to the dissociation threshold, the anisotropy of the angular distribution is reduced. When laser wavelengths become larger than the threshold, NO2 dissociates via two-photon absorption, and the photofragment angular distribution again becomes anisotropic reflecting the second photon absorption.

Photochemical oxidation of (Hg, Cd)Te: Passivation processes and characteristics

We have demonstrated the controlled growth of photochemical native oxide on (Hg, Cd)Te by the UV photodissociation of N2O. The initial growth rate is ∼7Å/min, is insensitive to temperature over the range from 40 to 100 °C, and is dependent on the surface Hg concentration. Encapsulation of the native oxide with photochemical SiO2 results in a degradation of the (Hg, Cd)Te–native oxide interface electrical properties. The presence of photochemical HgO between the native oxide and SiO2 results in superior (Hg, Cd)Te surface electronic properties. However, MOS structures comprising SiO2 on a double layer of HgO on photochemical native oxide undergo an electrical degradation at room temperature, likely owing to reactions between HgO and the native oxide or (Hg, Cd)Te substrate.

Proposal for vacuum-ultraviolet anti-Stokes Raman lasers based on the group VI elements

Metastable inversions in the(1)S(0) state of the group VI elements (oxygen, sulfur, and selenium) are proposed as host media for a new class of anti-Stokes Raman laser. Selective photodissociation of N(2)O, OCS, or OCSe with appropriate VUV wavelengths can create (1)S(0) inversion densities in excess of 10(16) atoms/cm(3). Using this population inversion to upconvert tunable input lasers should result in the creation of efficient, high-power lasers over the 100-206-nm spectral region.

Calculations of the temperature dependence of the NO2 photodissociation coefficient in the atmosphere

The photodissociation coefficient, J NO2 of NO2 in the atmosphere was calculated at 235 and 298 K using the measured temperature dependences of the absorption cross-sections and quantum yields. These calculations gave a ratio J NO2(298 K)/J NO2(235 K)=1.155±0.010 which is only weakly dependent on altitude, surface albedo and solar zenith angle.

Nascent NO vibrational distribution from 2485 Å NO2 photodissociation

The initial NO vibrational level distribution has been determined for NO2 photodissociation at 2485 Å. Excitation spectra of the NO vibrational levels were measured by using both the NO A 2Σ+←X 2Π and B 2Π←X 2Π transitions, the latter being somewhat stronger due to saturation effects. It was determined that the NO population was strongly inverted, with most of the nascent NO being in v=6–8; the thermodynamic limit is v=8. Injection locking of the KrF laser output permitted study of the 2491 Å NO2 band, and it was evident that the increased absorption in this region gave greatly enhanced signal levels in the excitation spectra, at those wavelengths where NO2 and NO absorption lines coincide. It was demonstrated that in the 2640–2850 Å wavelength region, NO2 can be detected by use of a single dye laser, simultaneously dissociating NO2 and electronically exciting the resultant vibrationally hot NO. Deactivation of NO(v=8) by NO2 was found to proceed with a rate coefficient of 1.1×10−11 cm3 molecule−1 s−1, whereas the coefficient for quenching by N2 and He was ≤2×10−15 cm3 molecule−1 s−1. The peculiar NO rotational distributions noted by Zacharias et al. in their study of NO2 dissociation at 3371 Å were also observed in the present work.

Dynamics of the two-photon photodissociation of NO2: A molecular beam multiphoton ionization study of NO photofragment internal energy distributions

Two-photon photodissociation of NO2 is induced by the output of a pulsed dye laser tuned over the region from 455 to 425 nm. We characterize the dynamics of this process by recording the multiphoton ionization spectrum of the product NO: intensities of spectral features associated with Ã(2Σ+)←X̃(2π3/2,1/2) two-photon resonance enhanced, four-photon ionization of nascent NO reveal its distribution over accessible rovibronic states. A single laser pulse serves both as photodissociation source and probe. Over the wavelengths studied the dominant reaction pathway yields NO(X̃ 2π) and O(1D). Its dynamics in all regions of the photodissociation spectrum, save one, are comparable to those observed for the loss of O(3P) in one photon photolysis at comparable available excess energy. In the region of 427 nm, however, the photodissociation dynamics are dramatically different. Here we find that the photoproduct NO rotational distribution is anomalously cold, apparently limited by the rotational temperature of our supersonic molecular beam, and that a product spin-orbit state 2π1/2 is missing. We argue that this result suggests a linear or near-linear dissociation geometry which imparts very little torque to the departing NO photofragment and places strict symmetry requirements on its spin-orbit state. We offer an interpretation that traces the cause of this anomalous behavior to the participation at the two-photon level of a theoretically predicted linear state of NO2.

Fluorescence yields from photodissociation of NO2 at 1060–1620 Å

The photoabsorption and fluorescence cross sections of NO2 were measured using synchrotron radiation in the 1060–1620 Å region. The fluorescence starts to appear at 1620 Å, and its cross section increases towards shorter excitation wavelengths. The fluorescence excitation spectra show vibrational structure of Rydberg states similar to the absorption spectrum. The quantum yields for the production of fluorescence in the 1900–8000 Å region increase with decreasing photon wavelengths, and reach a plateau of ∼10% in the 1190–1240 Å region. At photon wavelengths shorter than 1190 Å, the quantum yields increase to about 18% and show structure. Correlations of the absorption and fluorescence spectra with electronic states, as well as the photodissociation processes, are discussed.

XeF2 photodissociation studies. I. Quantum yields and kinetics of XeF(B) and XeF(C)

Photodissociation of XeF2 with synchrotron light pulses (0.3 ns duration) has been used as the source of the XeF(B, C, and D) excited states. The time-resolved profiles of the intensity of the resulting fluorescence have been recorded and partially analyzed. Most of the measurements were made in the strong XeF2 absorption band between 145 and 175 nm. The absorption cross section was redetermined out to 210 nm, with a maximum value of (5.9±0.5)×10−17 cm2 at 158 nm. By comparison with O(1S) signals from N2O photodissociation, quantum yields for XeF B, C, and D state production were determined. Radiative lifetimes of (14±1) and (100±10) ns were found for the B and C states. Rate coefficients for quenching by XeF2 are reported as are those for converting B to C by collision with Ne, Ar, and N2, along with upper limits for quenching of the C state by these gases.