Excited NO Molecules Research Articles

Quantum mechanical investigations of the N(S4)+O2(XΣg−3)→NO(XΠ2)+O(P3) reaction

The reaction between energetic nitrogen atoms and oxygen molecules has received important attention in connection with nitric oxide chemistry in the lower thermosphere. We report time-independent quantum mechanical calculations of the N(S4)+O2→NO+O reaction employing the XA′2 and aA′4 electronic potential energy surfaces of Sayós et al. [J. Chem. Phys. 117, 670 (2002)]. We confirm the production of highly vibrationally excited NO molecules, consistent with previous semiclassical and more recent time-dependent quantum wave packet studies. Calculations are carried out for total angular momentum quantum number J=0 and cross sections and rate coefficients are extracted using the J-shifting approximation. The results are in good agreement with available experimental and theoretical data.

Vibrationally promoted electron emission from low work-function metal surfaces

We observe electron emission when vibrationally excited NO molecules with vibrational state v, in the range of 9 < or = v < or =18, are scattered from a Cs-dosed Au surface. The quantum efficiency increases strongly with v, increasing up to 10(-2) electrons per NO (v) collision, a value several orders of magnitude larger than that observed in experiments with similar molecules in the ground vibrational state. The electron emission signal, as a function of v, has a threshold where the vibrational excitation energy slightly exceeds the surface work function. This threshold behavior strongly suggests that we are observing the direct conversion of NO vibrational energy into electron kinetic energy. Several potential mechanisms for the observed electron emission are explored, including (1) vibrational autodetachment, (2) an Auger-type two-electron process, and (3) vibrationally promoted dissociation. The results of this work provide direct evidence for nonadiabatic energy-transfer events associated with large amplitude vibrational motion at metal surfaces.

Differential cross sections for collisions of hexapole state-selected NO with He

The first measurements of differential inelastic collision cross sections of fully state-selected NO (j=12, Omega=12, epsilon= -1) with He are presented. Full state selection is achieved by a 2 m long hexapole, which allows for a systematic study of the effect of parity conservation and breaking on the differential cross section. The collisionally excited NO molecules are detected using a resonant (1+1') REMPI ionization scheme in combination with the velocity-mapped, ion-imaging technique. The current experimental configuration minimizes the contribution of noncolliding NO molecules in other rotational states j, Omega, epsilon--that contaminates images--and allows for study of the collision process at an unprecedented level of detail. A simple method to correct ion images for collision-induced alignment is presented as well and its performance is demonstrated. The present results show a significant difference between differential cross sections for scattering into the upper and lower component of the Lambda-doublet of NO. This result cannot be due to the energy splitting between these components.

Role of Vibrationally Excited NO in Promoting Electron Emission When Colliding with a Metal Surface: A Nonadiabatic Dynamic Model

A nonadiabatic quantum dynamic model has been developed to study the process of electron emission from a low-work-function metal surface. The process is initiated by scattering a highly vibrationally excited NO molecule from a surface composed of a Cs layer covering a Ru crystal. The model addresses the increasing quantum yield of the electron emission as a function of the molecular vibrational excitation and incident kinetic energy. The reaction mechanism is identified as a long-range harpooning electron transfer to a molecular ion which is then accelerated toward the surface. Upon impact, the molecular ion emits its excess electron.

Vibrationally promoted emission of electrons from low work function surfaces: Oxygen and Cs surface coverage dependence

We observe electron emission from low work function Cs covered Au surfaces due to the scattering of vibrationally excited NO molecules with 18 quanta of vibration in the ground electronic state. Additional experiments explore the influence of oxygen exposure on the electron emission. These results indicate a nondissociative mechanism for reactivity of vibrationally excited NO on these surfaces and provide evidence against an Auger-like mechanism. We note a remarkable similarity between trends of the vibrationally produced electron emission as the surface composition changes and reports on surface work function.

Conversion of large-amplitude vibration to electron excitation at a metal surface

Gaining insight into the nature and dynamics of the transition state is the essence of mechanistic investigations of chemical reactions, yet the fleeting configuration when existing chemical bonds dissociate while new ones form is extremely difficult to examine directly. Adiabatic potential-energy surfaces--usually derived using quantum chemical methods that assume mutually independent nuclear and electronic motion--quantify the fundamental forces between atoms involved in reaction and thus provide accurate descriptions of a reacting system as it moves through its transition state. This approach, widely tested for gas-phase reactions, is now also commonly applied to chemical reactions at metal surfaces. There is, however, some evidence calling into question the correctness of this theoretical approach for surface reactions: electronic excitation upon highly exothermic chemisorption has been observed, and indirect evidence suggests that large-amplitude vibrations of reactant molecules can excite electrons at metal surfaces. Here we report the detection of 'hot' electrons leaving a metal surface as vibrationally highly excited NO molecules collide with it. Electron emission only occurs once the vibrational energy exceeds the surface work function, and is at least 10,000 times more efficient than the emissions seen in similar systems where large-amplitude vibrations were not involved. These observations unambiguously demonstrate the direct conversion of vibrational to electronic excitation, thus questioning one of the basic assumptions currently used in theoretical approaches to describing bond-dissociation at metal surfaces.

Hexapole transport and focusing of vibrationally excited NO molecules prepared by optical pumping

Abstract We present a detailed report of experiments where hexapole focusing is combined with stimulated emission and Franck–Condon optical pumping techniques on a molecular beam. We show that this approach allows one to achieve a high degree of control over the molecule's ro-vibronic quantum numbers, its laboratory frame velocity, and its transverse divergence. Specifically, new ways of manipulating beams of vibrationally excited molecules emerge, including: (1) transverse refocusing and concomitant improved efficiency for transport of the vibrationally excited molecules, (2) relative enrichment of the concentration of the vibrationally excited molecules with respect to the unexcited portion of the beam, and (3) orientation of vibrationally excited molecules. Laser induced fluorescence, fluorescence depletion, Franck–Condon pumping, stimulated emission pumping, resonance enhanced multi-photon ionization, and hexapole focusing all provide spectroscopic probes into the detailed characteristics of this experiment.

Observation of diffractive orbits in the spectrum of excited NO in a magnetic field

We investigate the experimental spectrum of excited NO molecules in the diamagnetic regime and develop a quantitative semiclassical framework to account for the results. We show the dynamics can be interpreted in terms of classical orbits provided that in addition to the geometric orbits, diffractive effects are appropriately taken into account. We also show how individual orbits can be extracted from the experimental signal and use this procedure to reveal the first experimental manifestation of inelastic diffractive orbits.

Transport and focusing of highly vibrationally excited NO molecules

We report experiments where hexapole focusing is combined with stimulated emission pumping in a molecular beam, providing control over the molecule’s rovibronic quantum numbers, its laboratory frame velocity and its transverse divergence. Hexapole focusing profiles can be quantitatively reproduced by classical trajectory simulations. These experiments provide new ways of manipulating beams of vibrationally excited molecules including: (1) transverse refocusing and concomitant improved efficiency for transport of the vibrationally excited molecules, (2) relative enrichment of the concentration of the vibrationally excited molecules with respect to the unexcited portion of the beam and, (3) orientation of vibrationally excited molecules.

Dissociation dynamics from a de Broglie–Bohm perspective

Within the framework of Bohm’s reformulation of quantum physics we revisit the activated dissociation of hydrogen molecules at metal surfaces. The quantum-mechanical force, which accounts for most of quantum effects in the method, and is caused by nonlocal, topographical properties of the wave function, is computed using time-dependent wave packets obtained using conventional, spectral methods. Driven by a combination of the classical force together with the quantum force, trajectories carrying probability density either succeed in overcoming the barrier for dissociation or are scattered back into the gas phase. The Bohmian picture for the dissociation process has enabled us to develop a novel mechanism to account for vibrationally enhanced molecular dissociation. This is relevant to the recently observed promotion of dissociation of very highly vibrationally excited NO molecules at Cu surfaces.

Determination, through titration with NO, of the concentration of oxygen atomsin the flowing afterglow of Ar-O2 and N2-O2 plasmas used for sterilizationpurposes

Les méthodes existantes de titrage de N et O d'une post-décharge aumoyen de l'intensité d'émission de la molécule NO excitée ne permettant pasd'aller au-delà de x = 5% dans un mélange xO2-(100%-x)N2, nous présentons unedémarche valable pour x⩽20%. Cette technique est fondée sur la mesure del'intensité d'émission de NO2(A), en fonction du débit de NO introduit, enrelation avec une dérivation analytique des équations des concentrations [N]et [O]. La concentration d'oxygène atomique obtenue par cette méthode estvalidée de façon indépendante à partir de la mesure du rapport des intensitésd'émission de NO(B) et de N2(B, 11) (celle-ci détectable pour x⩽8%). Enfin,la méthode proposée est mise en oeuvre pour apprécier l'influence de la valeurde la concentration d'oxygène atomique sur le temps de stérilisation dans unepost-décharge en flux à partir d'un plasma de N2-O2.\\engabstract Existing titration methods of N and O in an afterglow based on theemission intensity of the excited NO molecule cannot be used at x valuesexceeding 5% in the xO2-(100%-x)N2 mixture. Our technique extends the x rangeto 20%. It utilizes the emission intensity measurement of NO2(A), as afunction of the introduced NO flow, in relation with analytically derivedequations for the O and N concentrations. The atomic oxygen concentrationobtained in this way is validated independently through measurements of theemission intensity ratio of NO(B) and N2(B, 11) (detectable for x⩽8%).Finally, the proposed method is used to assess the influence of the oxygenatom concentration on the sterilization time in the flowing afterglow of anN2-O2 plasma.

The dependence of initial states on the excitation of NOmolecules by chirped infrared laser pulses

We study the effect of initial states on the chirping excitationof NO molecules in order to interpretrecent experimental data. Theresults show that excitation is efficient when theprobability density localizesalong the polarization direction ofthe external field.Therefore, the thermal effect on the density distribution mustbe taken into account for the experimental case of 15 K.Furthermore, as the adiabatic limit is fully satisfied, theinterference pattern of excited state populationsdisappears. This becomes a limiting factor underexperimentalconditions. We also find that excitation can be enhancedsignificantly when the pulse intensity is increased to fit theadiabatic criterion.

Efficient production of vibrationally excited NO using a pulsed molecular beam discharge source

An efficient way of producing vibrationally excited, but rotationally cold NO molecules is described. A dc-discharge is incorporated into a piezoelectric pulsed molecular beam valve of the Gerlich design. Efficiencies larger than 10% are achieved for the production of vibrationally excited NO( v′′=1) characterized by a rotational temperature of 4 K. At lower backing pressures, a population ratio NO( v′′=1)/NO( v′′=0)=0.5 is found with rotational temperatures around 30 K. Population of NO in higher vibrational states, e.g. v′′=2, is generated with smaller but still significant density. Even highly vibrationally excited NO molecules with 7⩽ v′′⩽19 have been detected.

High-resolution laser spectroscopy of NO2 just above the X̃2A1–Ã2B2 conical intersection: Transitions of K−=1 stacks

The complexity of the absorption spectrum of NO2 can be attributed to a conical intersection of the potential energy surfaces of the two lowest electronic states, the electronic ground state of 2A1 symmetry and the first electronically excited state of 2B2 symmetry. In a previous paper we reported on the feasibility of using the hyperfine splittings, specifically the Fermi-contact interaction, to determine the electronic ground state character of the excited vibronic states in the region just above the conical intersection; 10 000 to 14 000 cm−1 above the electronic ground state. High-resolution spectra of a number of vibronic bands in this region were measured by exciting a supersonically cooled beam of NO2 molecules with a narrow-band Ti:Sapphire ring laser. The energy absorbed by the molecules was detected by the use of a bolometer. In the region of interest, rovibronic interactions play no significant role, with the possible exception of the vibronic band at 12 658 cm−1, so that the fine- and hyperfine structure of each rotational transition could be analyzed by using an effective Hamiltonian. In the previous paper we restricted ourselves to an analysis of transitions of the K−=0 stack. In the present paper we extend the analysis to transitions of the K−=1 stack, from which, in addition to hyperfine coupling constants, values of the A rotational constants of the excited NO2 molecules can be determined. Those rotational constants also contain information about the electronic composition of the vibronic states, and, moreover, about the geometry of the NO2 molecule in the excited state of interest. The results of our analyses are compared with those obtained by other authors. The conclusion arrived at in our previous paper that determining Fermi-constants is useful to help characterize the vibronic bands, is corroborated. In addition, the A rotational constants correspond to geometries that are consistent with the electronic composition of the relevant excited states as expected from the Fermi-constants.

Studies of the Spectral Dependence of Photoionization Spectrometry Measurements of NO in Air

The relative merits of five laser excitation schemes have been evaluated for photoionization spectrometry (PIS) measurements of NO in air. All five schemes utilize wavelengths near 215 nm, which correspond to excitation of rovibronic transitions in the A 2Σ+–X 2πi (1,0) bands of NO. Photoionization of the excited NO molecules is accomplished by using wavelengths ranging from 215 to 1064 nm. Excitation spectra of the five PIS schemes reveal a significant enhancement when 355 nm radiation is used for photoionization. The enhancement is hypothesized to be related to a resonance or near-resonance ionization process for selected lines in the A 2Σ+–X 2π1/2 band [specifically the R11 + Q21 ( J″ = 8.5–12.5) lines and the P21 + Q11 ( J” 9.5–10.5) lines] and the A 2Σ+–X 2π3/2 band [specifically the R12 + Q22 ( J” 8.5–12.5) lines and the P22 + Q12 ( J” = 9.5–10.5) lines] of NO. Comparison measurements using all five schemes demonstrate that the highest sensitivity and highest signal-to-noise ratios are observed by using the 215 nm + 355 nm scheme, for which a limit of detection of 80 parts per trillion by volume for NO in air has been achieved.

Rotationally specific rates of vibration–vibration energy exchange in collisions of NO(X 2Π1/2,v=3) with NO(X 2Π,v=0)

Infrared ultraviolet double resonance (IRUVDR) experiments have been performed to investigate the rotational specificity of the vibrational–vibrational (V–V) exchange process, NO(X 2Π1/2,v=3,Ji)+NO(v=0)→NO(X 2Π1/2,v=2,Jf)+NO(v=1), for which the vibrational energy discrepancy corresponds to 55.9 cm−1. Radiation from an optical parametric oscillator was used to excite NO molecules into a specific rotational level (Ji) in the X 2Π, Ω=12, v=3 state. Laser-induced fluorescence (LIF) spectra of the (0,2) band of the A 2Σ+–X 2Π1/2 system were then recorded at delays corresponding to a fraction of a collision. From the relative line intensities, rate coefficients were determined for transfer of the excited NO molecule from the level X 2Π1/2, v=3, Ji to different final rotational levels (Jf) in the X 2Π1/2, v=2 state. Results are reported for Ji=3.5, 4.5, 7.5, 10.5, and 15.5. The data show a significant, though not strong, propensity for J to decrease by one; i.e., for ΔJ=Jf−Ji=−1, especially for the higher Ji levels. This result is interpreted as arising from a combination of (a) the tendency to minimize the energy that has to be accommodated in the relative translation of the collision partners, and (b) the favoring of ΔJ=±1 changes when V–V intermolecular exchange occurs under the influence of dipole–dipole interactions.

Photoinitiated unimolecular decomposition of NO2: Rotational dependence of the dissociation rate

Photoinitiated unimolecular decomposition rate constants of rotationally excited NO2 molecules have been measured near dissociation threshold (D0) by employing a double resonance technique. Rotational selectivity has been achieved by using narrow-linewidth (0.015 cm−1) infrared excitation to prepare specific rotational levels (N′=1,3,…,15, Ka′=0) of the (1,0,1) vibrational level. The picosecond-resolution pump–probe technique has then been used to photodissociate the molecules thus tagged and to monitor the appearance of the NO product. Data have been obtained for two progressions of average excess energies, 〈E〉−D0: (i) 10 cm−1+E101rot and (ii) 75 cm−1+E101rot, where 〈E〉 denotes an average over the pump laser linewidth and E101rot is the rotational energy of the (1,0,1) X̃ 2A1 intermediate vibrational level. The measured rate constants do not display any noticeable dependence on N′, which is a reflection of significant rovibronic interaction. Spin–rotation interaction, which has been implicated as the main source of rovibronic coupling for small values of N′, is not likely to yield such a result. A model is proposed to describe the influence of rotation on the dissociation rate. The experimental data are consistent with a Coriolis coupling mechanism causing transitions to occur between Ka levels.

The interaction of highly vibrationally excited molecules with surfaces: vibrational relaxation and reaction of NO(v) at Cu(111) and O/Cu(111)

We have studied the reaction and inelastic scattering of ground and vibrationally excited NO on Cu(111). We employed laser-based techniques to prepare NO in vibrationally excited states, stimulated emission pumping (SEP) to prepare v=13 and v=15 and infrared overtone pumping to prepare v=2. Laser ionization detection schemes were developed for probing the state distribution of highly vibrationally excited NO molecules. Ground-state NO(v=0) dissociates at Cu(111) with a probability of ≈2×10-4, with little dependence on the translational energy in the range between 29 and 65 kJ mol-1. The dissociation probability is strongly enhanced by vibrational excitation to v=13 and 15. The dissociation continues until the oxygen coverage on Cu(111) reaches saturation. For highly excited NO(v=13, 15) scattering from O/Cu(111), we have seen efficient multi-quantum relaxation (up to Δv=-5). For NO(v=2), in contrast, the survival probability is nearly 90%. Measurements of the translational and rotational state distributions after scattering support a direct-inelastic mechanism for vibrational relaxation, with strong flow of energy into the surface. The branching ratios for vibrational relaxation are independent of the kinetic energy in our experiments.

Remote Plasma‐Enhanced Chemical Vapor Deposition of SiO2 Using Ar/ N 2 O and SiH4

Remote plasma‐enhanced chemical vapor deposition of using a radio‐frequency (rf) Ar/ plasma and downstream‐injected was investigated. The deposition rate at 20 W rf power was measured as a function of pressure, temperature, and flow rate. The deposition rate at 300°C and 300 mTorr depends linearly on the flow rate. The deposition rate is independent of flow rate for ratios much greater than 1, consistent with oxygen saturation of the growth surface. The deposition rate increases linearly with pressure up to 400 mTorr. A plateau in the deposition rate is observed above 400 mTorr, and is ascribed to the onset of parasitic gas‐phase reactions leading to particle formation. Negative apparent activation energies are observed at pressures ⩽400 mTorr, suggesting that adsorption of Si‐bearing species is the rate‐limiting step in deposition. The deposition chemistry was probed using real‐time quadrupole mass spectrometry (QMS) and optical emission spectroscopy (OES). The and QMS signal intensities increase monotonically with flow rate; approximately 0.67 moles of and 1.33 moles of are produced per mole of consumed. OES evidences the presence of Ar metastables, metastables, excited NO molecules, and atomic O in the plasma. Fourier transform infrared spectroscopy of thick films demonstrated that Si‐H and Si‐OH groups are present at very low concentrations (<1 atom %). Single‐wavelength ellipsometry indicated that films deposited under typical O‐rich conditions have an average refractive index of 1.464.

Nanosecond time-resolved FTIR emission spectroscopy: Monitoring the energy distribution of highly vibrationally excited molecules during collisional deactivation

The 10−8 second time resolution in infrared emission spectroscopy has been demonstrated using a Fourier Transform spectrometer paired with a fast HgCdTe detector. The rapid time response of this system has enabled us to measure, with subcollisional period time resolution, the emission spectrum of highly vibrationally excited NO2 molecules during collisional deactivation by room temperature NO2. The greatly improved time resolution of the spectra allows the determination of N(E,t), the instantaneous energy distribution of the ensemble of excited molecules, with virtually no distortion due to collisional averaging. In addition, an improved procedure for extracting optimized N(E,t) from the spectral data makes no prior assumptions about the shape of the energy distribution. It is found that the distribution is well approximated as the sum of a Gaussian function at high vibrational energies and a population at low energies resulting from V–V transfer to bath NO2 molecules. The observation of a Gaussian-like function for the highly excited molecules is consistent with the widely invoked assumption that the step-size function of energy transfer per collision is exponential.