Lambda Doublet Components Research Articles

The ac Stark effect in nitric oxide induced by rapidly swept continuous wave quantum cascade lasers

A large ac Stark effect has been observed when nitric oxide, at low pressure in a long optical path (100 m) Herriot cell, is subjected to infrared radiation from a rapidly swept, continuous wave infrared quantum cascade laser. As the frequency sweep rate of the laser is increased, an emission signal induced by rapid passage occurs after the laser frequency has passed through the resonance of 1-0 R(11.5)(3/2 /)molecular absorption line. At very high sweep rates a laser field-induced splitting of the absorptive part of the signal is observed, due to the ac Stark effect. This splitting is related to the Autler-Townes mixing of the e, f lambda doublet components of the 1-0 R(11.5)(3/2) transition, which lie under the Doppler broadened envelope.

Suppression of nonadiabatic losses of molecules from chip-based microtraps

Polar molecules in selected quantum states can be guided, decelerated and trapped using electric fields created by microstructured electrodes on a chip. Here we explore how non-adiabatic transitions between levels in which the molecules are trapped and levels in which the molecules are not trapped can be suppressed. We use 12-CO and 13-CO (a 3-Pi(1), v=0) molecules, prepared in the upper Lambda-doublet component of the J=1 rotational level, and study the trap loss as a function of an offset magnetic field. The experimentally observed suppression (enhancement) of the non-adiabatic transitions for 12-CO (13-CO) with increasing magnetic field is quantitatively explained.

Measurements and Quasi-Quantum Modeling of the Steric Asymmetry and Parity Propensities in State-to-State Rotationally Inelastic Scattering of NO (2Π1/2) with D2

Relative integrated cross sections are measured for spin-orbit-conserving, rotationally inelastic scattering of NO (2Pi1/2), hexapole-selected in the upper Lambda-doublet level of the ground rotational state (j = 0.5), in collisions with D2 at a nominal energy of 551 cm-1. The final state of the NO molecule is detected by laser-induced fluorescence (LIF). The state-selected NO molecule is oriented with either the N end or the O end toward the incoming D2 molecule by application of a static electric field E in the scattering region. This field is directed parallel or antiparallel to the relative velocity vector v. Comparison of signals taken for the different applied field directions gives the experimental steric asymmetry SA, defined by SA = (sigma v upward arrow downward arrow E - sigma v upward arrow upward arrow E)/(sigma v upward arrow downward arrow E + sigma v upward arrow upward arrow E), which is equal to within a factor of -1 to the molecular steric effect, Si-->f identical with (sigmaD2-->NO - sigmaD2-->ON)/(sigmaD2-->NO + sigmaD2-->ON). The dependence of the integral inelastic cross section on the incoming Lambda-doublet component is also measured as a function of the final rotational (jfinal) and Lambda-doublet (epsilonfinal) state. The measured steric asymmetries are similar to those previously observed for NO-He scattering. Spin-orbit manifold-conserving collisions exhibit a larger propensity for parity conservation than their NO-He counterparts. The results are interpreted in the context of the recently developed quasi-quantum treatment (QQT) of rotationally inelastic scattering [Gijsbertsen, A.; Linnartz, H.; Taatjes, C. A.; Stolte, S. J. Am. Chem. Soc. 2006, 128, 8777]. The QQT predictions can be inverted to obtain a fitted hard-shell potential that reproduces the experimental steric asymmetry; this fitted potential gives an empirical estimate of the anisotropy of the repulsive interaction between NO and D2. QQT computation of the differential cross section using this simple model potential shows reasonable agreement with the measured differential cross sections.

Inelastic state-to-state scattering of OH (Π3∕22,J=3∕2,f) by HCl

Parity resolved state-to-state cross sections for inelastic scattering of OH (X2Pi) by HCl were measured in a crossed molecular beam experiment at the collision energy of 920 cm(-1). The OH (X2Pi) radicals were prepared in a single quantum state, Omega=3/2, J=3/2, MJ=3/2, f, by means of electrostatic state selection in a hexapole field. The rotational distribution of the scattered OH radicals by HCl was probed by saturated LIF spectroscopy of the 0-0 band of the A 2Sigma+ - X 2Pi transition. Relative state-to-state cross sections were measured for rotational excitations up to J=9/2 within the Omega=3/2 spin-orbit manifold and up to J=7/2 within the Omega=1/2 spin-orbit manifold. A propensity for spin-orbit conserving transitions was found, but no propensity for excitation into a particular Lambda-doublet component of the same rotational state was evident. The data are presented and discussed in comparison with results previously obtained for collisions of OH with CO (Ecoll=450 cm(-1)) and N2 (Ecoll=410 cm(-1)) and with new data we have measured for the OH+CO system at a comparable collision energy (Ecoll=985 cm(-1)). This comparison suggests that the potential energy surface (PES) governing the interaction between OH and HCl is more anisotropic than the PES's governing the intermolecular interaction of OH with CO and N2.

Degenerate four-wave mixing spectroscopy as a probe of photodissociation dynamics: nascent SH population distributions from the 266 nm photolysis of hydrogen sulfide

Degenerate four-wave mixing (DFWM) has been used to interrogate the nascent mercapto radicals (SH) emerging from photolysis of bulk-gas H2S samples at λdiss = 266 nm. These experiments reveal a ‘cold’ non-Boltzmann distribution of SH molecules formed exclusively in vibrationless levels of the X2Π ground electronic state, with a non-thermal population of spin-orbit states but no preference for lambda doublet components. Nearly all of the available energy appears as translational motion of the recoiling photofragments. Comparison with previous free-jet studies at λdiss = 193 nm shows the SH (ν = 0) rotational distribution to be relatively independent of photolysis wavelength and precursor rotational temperature.

Product state resolved stereodynamics of the reaction H(2S)+CO2OH(X2Π3/2; v = 0,N= 5) CO(1Σ+)

The detailed stereodynamics of the H+CO2 system have been studied using polarized Doppler resolved laser induced fluorescence to state selectively probe the OH product. The lambda-doublet specific differential cross-sections and translational energy distributions were measured, along with the scattering angle dependence of the rotational angular momentum polarization. The observed centre-of-mass differential cross-section has significant contributions in both the forward and backward hemispheres, with a small preference for backward scattering, and the measured fraction of available energy released as translational energy is greater than that predicted by statistical models. The rotational angular mometum vector of the OH product displays a polarized angular distribution, measured with respect to the reagent relative velocity vector k and the kk' scattering plane, with the π(A'') lambda-doublet component being significantly more strongly polarized than the π(A') level. In addition, those products scattered backwards are seen to have a different polarization from those scattered forwards. The unprecedented detail of the information obtained is explained in terms of rather stringent steric constraints in the entrance channel and the formation of short-lived collision complexes.

Reactions of velocity-aligned atoms probed by Doppler profiles: H+O2→OH+O

Final state resolved vector properties of the reaction H (2S)+O2(3Σ−g)→OH(2Π)+O(3P) have been measured using Doppler spectroscopy. Velocity-aligned H atoms were generated by photodissociation of H2S with linearly polarized ArF excimer laser light. The correlated velocity and angular momentum distribution of nascent OH reaction products was probed by laser induced fluorescence. Techniques are described for analyzing the line shapes, modeling the bipolar moments of the OH velocity and angular momentum distribution as parametric functions of the scattering angle. Distinctive Doppler-broadened lineshapes indicate scattering angle distributions with a strong forward and a weaker backward maximum. A qualitatively different angular momentum polarization for the two lambda doublet components of the OH product is required to explain the Doppler profiles.

Formation of lambda-doublet components of OH in the bimolecular reactions: H + OX → OH + X

The formation of lambda-doublet levels in the OH( 2Π) radical produced in chemical reactions of the type H + OX → OH + X has been examined on the basis of a classical model. An examination of the hybridization in the collision complex leads to even populations for the lambda-doublet components in the case of no dynamical preference, in contrast to the previous treatment. Examples of the dynamical preference leading to deviation from even populations are discussed in relation to the existing experimental data.

266 nm Photolysis of HOCl and DOCl: laser-excitation fluorescence detection of OH and OD

Fragment rotational population distributions for OH and OD were observed by laser-excitation fluorescence following the 266 nm photolysis in a free jet molecular beam of HOCl and DOCl, respectively. The photodissociation dynamics show a strong preference for the 2Π(A′) OH or OD lambda doublet component in both spin–orbit states. The 2Π3/2 state is more populated than the 2Π1/2 state of OH. The distributions are consistent with photolysis via a repulsive excited state and the measured rotational alignment (–0.28) is compatible with an excited-state symmetry of A′ for the dominant dissociation pathway. The high-J limiting value for the degree of electron alignment reaches the extreme value of –1 for the 2Π1/2 state for both OH and OD, but falls short of this for the 2Π3/2 state, reaching only –0.7.

An experimental and theoretical study of shape resonances in CH+

A study of shape resonance energies and widths in the A 1Π state of 12CH+ and 13CH+ is reported. The experimental data were obtained by high-resolution laser photofragment spectroscopy of CH+ in a fast ion beam. Spectroscopic transitions were recorded by tunable laser excitation from levels of the X 1Σ+ state to shape resonances (quasibound rotational levels) of the A 1Π state and detection of photoproduct C+ ions. Some of the dissociation lifetimes are sufficiently short to cause broadening of the spectral lines. The linewidths are interpreted in terms of tunnelling through the centrifugal barrier and the effects of rotational-electronic and radial coupling. The e lambda-doublet component in two of the resonances is found to have a narrower linewidth than the corresponding f component. This is shown to be due to the effect of the rotational-electronic (Coriolis) interaction on the shapes of the rotationally adiabatic potential-energy curves which determine the tunnelling probability. The contributions to the predissociation rates are calculated and are in good agreement with the experimental results.

Nitric oxide photofragment rotational distribution from UV dissociation of cyclopentadienylnickel nitrosyl

The rotational state distribution for NO produced by dissociation of (η5–C5H5)NiNO near 225 nm has been obtained by [1+1] resonance-enhanced multiphoton ionization. The distribution is approximately characterized by a Boltzmann distribution at T=900±200 K. The relative populations of the two spin–orbit components of the ground 2Π manifold are also consistent with this temperature. No preference is observed for populating either of the lambda–doublet components, even at high rotational quantum numbers. Comparison of the results with statistical models shows that the state distribution is significantly colder than expected. One plausible explanation for this is that the parent molecule dissociates to produce cyclopentadienylnickel fragments in an excited electronic state.

The 355 nm Photolysis of Methyl Nitrite

Single photon laser induced fluorescence (LIF) has been used to study the NO fragment produced in its ground electronic state from the 355 nm photolysis of CH3ONO. Populations of rotational, vibrational, spin-orbit and lambda doublet components have been measured. The results are in broad agreement with previous two photon LIF studies, and confirm the dynamics of the process in which the NO fragment departs from an essentially planar CONO framework with non-statistical energy partitioning in the internal states of the products.

Infrared multiphoton dissociation of methyl nitrite in a molecular beam: Internal states of the nitric oxide fragment

The rotational-, spin-, and lambda doublet-state distributions for nitric oxide (NO) formed in the CO2 laser multiphoton dissociation of methyl nitrite, CH3ONO, in a pulsed molecular beam are reported. Upon methyl nitrite photolysis by temporal square wave infrared laser pulses at 983 cm−1 of 50 ns duration and 800 MW/cm2 intensity, the low-lying rotational levels of the nitric oxide fragments formed in the 2Π1/2 (F1) and 2Π3/2 (F2) spin-orbit states exhibited Boltzmann-like population distributions, characterizable by the rotational temperatures TR (F1)=400±10 K and TR (F2)=530±100 K; the integrated populations for J<30.5 of the two spin components were in the ratio F1/F2=2.7 : 1. For those highly rotationally excited levels with J≳24.5 there is no measurable spin preference, the level population depending solely on total internal energy Eint. There is no apparent preference for formation of either lambda doublet component and there is no observable fragment alignment, the nascent NO species exhibiting an isotropic distribution of angular momentum vectors.

Neutral particle density modulation in the positive column of a low-pressure mercury discharge modulated at low frequencies: JR Riley, J Phys D, Appl Phys, 3 (8), 1970, 1226–1239

Degenerate four-wave mixing (DFWM) has been used to interrogate the nascent mercapto radicals (SH) emerging from photolysis of bulk-gas H2S samples at λdiss = 266 nm. These experiments reveal a ‘cold’ non-Boltzmann distribution of SH molecules formed exclusively in vibrationless levels of the X2Π ground electronic state, with a non-thermal population of spin-orbit states but no preference for lambda doublet components. Nearly all of the available energy appears as translational motion of the recoiling photofragments. Comparison with previous free-jet studies at λdiss = 193 nm shows the SH (ν = 0) rotational distribution to be relatively independent of photolysis wavelength and precursor rotational temperature.