The use of rovibrationally excited NO photofragments as trace nitrocompounds indicators

A one-color laser performs photolysis of nitrocompounds and laser-induced fluorescence to monitor the generated NO photofragments and to sensitively detect vapor trace amounts of nitrocompounds. The NO is monitored via excitation and emission through A²Σ⁺(v^′=0)?X²Π(v^′′=2-0) and A²Σ⁺(v^′=0)?X²Π(v^′′=0,1) transitions, respectively. It is found that NO photofragments populate the vibrationless ground state and also the first two vibrationally excited states. The analytical performance of the method is demonstrated on 2,4 dinitrotoluene (DNT) via excitation through A²Σ⁺(v^′=0)?X²Π(v^′′=2). The achieved limits of detection are 3.7 and 2.7 parts per billion (ppb) by weight of gaseous DNT in 100 and 500 Torr of air, for 30 s integration time. The application of this scheme for trace nitrocompound detection has the advantage that no background of ambient ground-state NO interferes and that the fluorescence is collected at shorter wavelengths than the exciting radiation, precluding background fluorescence. Received: 18 August 1999 / Revised version: 22 November 1999 / Published online: 8 March 2000     

2-D absolute OH concentration profiles in atmospheric flames using planar LIF in a bi-directional laser beam configuration

1 (6) rotational line in the A²Σ⁺(v^′=0)←X²Π(v^′′=0) band of OH at 309 nm. The requirements for obtaining a good signal-to-noise ratio for the technique are discussed and the possibilities of single-shot measurements are investigated. Received: 31 October 1996/Revised version: 3 December 1996     

Determination of the helium-4 mass in a Penning trap

The determination of the rotational quadrupole alignment of diatomic molecules via REMPI detection is investigated. In this process a high focal intensity usually increases the detection probability. At high intensities the AC Stark effect may cause a splitting of the normally degenerate m_J sublevels of a rotational state J beyond the spectral width of the exciting radiation. This leads to a selective detection of only certain m_J states with the consequence that deduced alignment factors can be misleading. From the theoretical considerations line profiles are explicitly calculated for dynamic polarizabilities which represent the B ¹Σ⁺ _u←X ¹Σ⁺ _g transition of H₂, in order to fit an experimental (3+1) REMPI spectrum and to predict (1+1') line shapes as a function of laser intensity. It is further shown that the deduced quadrupole alignment factor A ₀ ⁽²⁾ is significantly changed by the second order AC Stark effect when the intensities are chosen high enough to observe asymmetric broadened line profiles. Different combinations of relative linear polarizations of the exciting and ionizing laser beams are discussed. Received 1st August 2000 and Received in final form 2 May 2001     

Collisional processes near the CH B 2Σ- v′=0,1 predissociation limit in laser-induced fluorescence flame diagnostics

Excitation and dispersed laser-induced fluorescence spectra of CH B ²Σ^-v^′=0,1 in methane flames are analyzed using rotational relaxation models to investigate their applicability for flame diagnostics. The existence of non-predissociative and highly predissociative rotational levels in the same vibrational state provides a unique scenario to test the effects of rotational relaxation in laser-induced fluorescence measurements. Using a statistical power gap law for rotational relaxation modeling, we find that the levels with collision-free lifetimes as short as 100 ps have apparent fluorescence yields larger than expected because of the extent of rotational relaxation at atmospheric pressure. Also, vibrational (v^′=1 to v^′=0) and electronic energy transfer (B ²Σ^-v^′=1 to A ²Δ) are competitive, and together are half the value for the total collisional removal rate from CH B ²Σ^-v^′=0. The measured electronic energy transfer branching ratio into A (v^′=0-3) depends on the initial rotational level pumped, and energy gap considerations can be used to explain these propensities. The combination of measurements and model calculations finds the excitation of the CH B ²Σ^- v^′=1,N^′=8 level a good candidate for laser-induced fluorescence quantitative measurements in flames at atmospheric pressure. Received: 24 September 1999 / Published online: 7 June 2000     

Molecular relaxations in mixtures of O2 with CO2 observed on?laser-induced gratings

Laser-induced gratings in mixtures of O₂ with CO₂ were formed by excitation of the O₂ molecules to the singlet state b ¹ Σ _g ⁺(v′=0). Density changes from heat release and from electrostriction, and variation of electric polarizability by excitation of molecules contribute to the grating. For modeling the relaxation of the excited O₂ molecules, a three-step process is assumed: Fast heat release with respect to the rotational states first, then medium fast electronic de-activation with excitation of vibrational states of the O₂ and CO₂ molecules, and then the final slow heat release processes. The observed temporal evolution of the diffraction efficiency of the grating agrees rather well with modeling. Average rates of the final relaxations and the value of the polarizability of the singlet state b ¹ Σ _g ⁺(v′=0) are determined in this way.     

Visualisation of a supersonic free-jet expansion using laser-induced fluorescence spectroscopy: Application to the measurement of rate constants at ultralow temperatures

2 Σ⁺, v^′=0) by air at 26±4 K. A value of 2.56±0.40×10^-10 molecule^-1 cm³ s^-1 was obtained, a factor of more than four higher than at room temperature, and consistent with attractive forces dominating the quenching of OH(A²Σ⁺). A d etailed computational fluid dynamics (CFD) simulation of the supersonic free-jet expansion was performed, providing a two-dimensional visualisation of temperature and density variations throughout the expansion. The CFD calculations reproduced the salient features of the experimental temperature and density profiles along the centreline. Comparison between experiment and computation has allowed validation of CFD codes. Received: 31 January 1997/Revised version: 24 March 1997     

Radiative lifetimes of the NaRb C(3)1Σ+ state: experiment and theory

Radiative lifetimes for 2≤v^′≤44 rovibronic C¹Σ⁺ state levels of NaRb and quenching collision cross-sections with Rb atoms have been directly measured in a thermal cell by detecting time resolved laser induced fluorescence after pulsed excitation. Many body multipartitioning theory was applied to calculate C¹Σ⁺-X¹Σ⁺ and C¹Σ⁺-A¹Σ⁺ transition dipole moments. The relevant ab initio matrix elements were converted to the C¹Σ⁺ state radiative lifetimes. The strong spin-orbit A¹Σ⁺∼ b³Π coupling effect on the total C → A transition probabilities and lifetimes of the C¹Σ⁺ state is discussed. The measured radiative lifetimes show a decrease from 61 to 34 ns as the v^′ values increase, the results being in good agreement with calculations. The averaged collisional quenching cross-section value σ=(3±1)×10^-14 cm² was determined for NaRb (C¹Σ⁺) + Rb collisions from the Stern-Volmer plots.     

Measurement of vibrational energy transfer of OH (A2Σ+,v′=1→0) in low-pressure flames

2 Σ⁺) was measured in a low-pressure H₂/O₂ flame for three rotational levels of OH (v^′=1). Rate coefficients for collisions with H₂O and N₂ were determined. At 1600 K, k_VET (N₂) is (in 10^-11 cm³s^-1) 10.1±2, 6.1±1.8, and 3.8±1.3 for N^′=0, 5, and 13, respectively. The k_VET (H₂O) is <1.1±1.8. The k_Q (N₂) is <2.4±8 for both vibrational levels. The k_Q (H₂O) in v^′=1 is 59.1±6.5, 54.7±6.4, and 54.9±6.6 for N^′=0, 5, and 13, respectively, and, determined indirectly, 74.6±10.4, 70.6±10.3, and 63.4±7.3 for N^′=0, 5, and 13 in v^′=0. A multi-level model of OH population dynamics, which is being developed for the quantitative simulation of experimental LIF spectra, was extended to include VET. It was attempted to simulate state-to-state-specific VET coefficients for N₂ collisions. From these simulations it appears that angular momentum conservation does not determine the N dependence of the vibrational relaxation step. Received: 9 September 1996/Revised version: 6 January 1997     

High-resolution F T spectrum of A2Πr-X2Σ+ band system of MgCl

The emission spectrum of the A ² Π _r -X ² Σ ⁺ band system of MgCl molecule (360–380 nm) has been recorded on BOMEM DA8 Fourier transform spectrometer at an apodized resolution of 0.035 cm⁻¹. The spectra have been excited under flowing conditions in a demountable stainless steel hollow cathode lamp (400 V, 250 mA) containing anhydrous MgCl₂ and Ar. The resulting spectra are very intense and the 0-0, 1-1, 1-0 bands of A ² Π _1/2-X ² Σ ⁺ sub-transition and the 0-0 band of A ² Π _3/2-X ² Σ ⁺ sub-transition have been rotationally analyzed. Improved molecular constants have been derived using a least-squares fit program in which optical data of earlier analyzed 0-1 and 0-2 bands (A ² Π _1/2-X ² Σ ⁺) was also included. The Λ-doubling constants in the v′ = 0, 1 levels of the A ² Π _1/2 sub-state are as expected, i.e. p ₁ > p ₀, where as it is found that the spin-doubling constants of the v″ = 0, 1 and 2 levels of the ground state X ² Σ ⁺, decrease with the increase in v, i.e. γ ₀ > γ ₁ > γ ₂. This is indicative of the presence of some nearby state, influencing the spin-doubling.     

Molecular oxygen detection in low pressure flames using cavity ring-down spectroscopy

Cavity ring down spectroscopy is used for measurement of the concentration profiles of oxygen in the low pressure (30 Torr) methane/nitrogen/oxygen flames. Three different equivalence ratios are used: 0.8, 1.0 and 1.17. Molecular oxygen concentration is monitored via rotational spectrum of b¹ Σ _g ⁺←X³ Σ _g ^- (v^′=0-v^′′=0) transition, also known as atmospheric A band, located near 750 nm. The P(15)P(15) line is used for concentration measurements. The sensitivity reached is 2.2×10^-8 cm^-1. The concentration profiles are in a good agreement with the ones calculated using GRI-3.0 mechanism. PACS 33.20.Kf; 33.70.Fd; 42.60.Da     

Parametric four-wave mixing in the 3d and 4d Rydberg states of NO

2 Σ⁺,H^′ 2Π^±(v^′=2), and 4dσ,πO²Σ⁺,O^′ 2Π^±(v^′=0) Rydberg states of NO molecules are described. The analyses of the two-photon excitation functions and infrared emission spectra revealed that only the ²Π^- component was involved in PFWM. This is in good accordance with the absence of amplified spontaneous emission (ASE) in the ²Σ⁺ and ²Π⁺ components due to their predissociative character. Results provide some support for the mechanism that the generation of the vacuum ultraviolet radiation was brought by PFWM in which ASE served as a third driving wave. Received: 6 January 1998     

High-spin states in odd-odd 168Lu

High-spin states in the odd-odd ¹⁶⁸Lu nucleus, populated in the ¹⁵⁴Sm(¹⁹F,5n) reaction at a beam energy of 96 MeV, were investigated using in-beam γ-ray spectroscopy techniques. The BC neutron crossing in the yrast band, based on πg _7/2[404]7/2⁺⊗νi _13/2[642]5/2⁺ configuration, occurs at ħω= 0.31 MeV. The two side bands, based on πh _11/2[514]9/2⁻⊗νi _13/2[642]5/2⁺ and πh _9/2[541]1/2⁻⊗νi _13/2[642]5/2⁺ configurations, show anomalous signature-splitting and signature-inversion in the first one, to occur at ħω= 0.24 MeV. A moderately delayed BC-crossing is anticipated in the second one. Received: 15 June 1998 / Revised version: 11 January 1999     

Dark resonances for ground-state transfer of molecular quantum gases

One possible way to produce ultra-cold, high-phase-space-density quantum gases of molecules in the rovibronic ground state is given by molecule association from quantum-degenerate atomic gases on a Feshbach resonance and subsequent coherent optical multi-photon transfer into the rovibronic ground state. In ultra-cold samples of Cs₂ molecules, we observe two-photon dark resonances that connect the intermediate rovibrational level |v=73,J=2〉 with the rovibrational ground state |v=0,J=0〉 of the singlet X ¹ Σ _g ⁺ ground-state potential. For precise dark resonance spectroscopy we exploit the fact that it is possible to efficiently populate the level |v=73,J=2〉 by two-photon transfer from the dissociation threshold with the stimulated Raman adiabatic passage (STIRAP) technique. We find that at least one of the two-photon resonances is sufficiently strong to allow future implementation of coherent STIRAP transfer of a molecular quantum gas to the rovibrational ground state |v=0,J=0〉.     

Parametric four-wave mixing in the H2Σ+,H′2Π(v′=0) states of NO

2 Σ⁺,H^′2Π(v^′=0)←X²Π(v^′′=0) two-photon transition of NO, both near-infrared and vacuum ultraviolet radiation were emitted along the laser propagation direction. The analyses of emission and excitation spectra revealed that the parametric four-wave mixing (PFWM) process coexisted with amplified spontaneous emission. Polarization properties of the IR radiation are found to be dependent on the rotational levels. Pressure and laser power behaviors of the generated waves were reported. The mechanism of PFWM was discussed in terms of selection rules of the relevant ro-vibronic transitions. Received: 19 September 1996/Revised version: 27 January 1997     

Intensity distribution in theA 2Π –X 2Σ+ system of Yttrium monoxide

The integrated intensities of a few bands in the vibrational structure of the astrophysically significant ‘orange’ system of the molecule yttrium monoxide, have been measured experimentally by the technique of photographic photometry. The effective vibrational temperatures of the source are found to be 2402±180 K and 2901±230 K for the subsystemsA ²Π_3/2−X ² Σ ⁺ andA ²Π_1/2−X ² Σ ⁺ respectively.     

The total yields of K *(892)+, Σ+(1385), and Σ0 in neutrino-induced reactions at ? E v? ≈ 10 GeV

Using the data obtained with the SKAT bubble chamber, the total yields of K*(892)⁺, Σ⁺(1385), and Σ⁰ are estimated for the first time in neutrino-induced reactions at moderate energy 〈E _v〉 = 10.4 GeV. It is shown that the recently observed enhancement of the K ⁰ and Λ yields in vA interactions as compared to vN interactions is contributed only slightly by the K*(892)⁺ and Σ⁺(1385) production. The contribution of resonances to the K ⁰ and Λ yields is found to be in qualitative agreement with higher energy (〈E _v〉 ≳ 40 GeV) data. It is shown that the energy dependence of the K*(892)⁺ mean multiplicity in vN interactions is approximately linear in the range of 〈E _v〉 ≈ 10−60 GeV, while that for Σ⁰ in vA interactions (A = 20−21) is approximately logarithmic in the range of 〈E _v〉 ≈ = 10−150 GeV. The text was submitted by the authors in English.     

Rotational analysis ofA 1Π-X 1Σ+ bands of P14N and P15N: Perturbatio studies in theA 1Π state

The ultraviolet band systemsA ¹Π-X ¹Σ⁺ of P¹⁴N and P¹⁵N were excited in an electrodeless tube containing traces of phosphorus specpure nitrogen and neon using a microwave discharge (2450 MHz). Bands of the isotopic species, P¹⁵N, were obtained using¹⁵N₂ enriched to 95.5%. Rotational analyses of eleven bands of P¹⁴N and sixteen bands of P¹⁵N were carried out. Three perturbing statese ³Σ⁻,d ³Δ andb ³Π, arising from the lower valence configurations were identified from the observed perturbations in thev′=0–4 levels of theA ¹Π state. Deperturbation studies led to the determination of molecular constants of the perturbing states. Vibrational assignments of the perturbing states were made from isotope shift studies.     

State selective step-wise photoionization of NO with mass spectroscopic ion detection

State and isotope selective two-step photoionization of NO with mass spectroscopic ion detection has been demonstrated and investigated. Using saturation condition the photoionization cross section for a single rotational level of the intermediate state, No(A ²Σ⁺, ν′=0), has been measured: σ_i=(7.0±0.9) X 10⁻¹⁹ cm². The charge transfer¹⁵NO⁺+¹⁴NO→¹⁵NO+¹⁴NO⁺ has been observed and investigated, yielding a cross section of the order of 13×10⁻¹⁶ cm², consistent with recent measurements at about 1 eV.     

New data on the p + N → [Σ0K+] + N reaction at E p= 70 GeV and the search for exotic baryons

New data for the diffractive reaction p+N→ [Σ⁰ K ⁺] +N at E _p= 70 GeV were obtained with partially upgraded SPHINX setup. The data are in a good agreement with the results of our previous study of this reaction. In the mass spectrum M(Σ⁰ K ⁺) a structure at the threshold region with a mass ∼1810 MeV and a distinct X(2000) peak with M= 1989 ± 6 MeV and γ= 91 ± 20 MeV are observed. Unusual features of the massive X(2000) state (narrow decay width, anomalously large branching ratio for the decay channel with strange particle emission) make it a serious candidate for cryptoexotic pentaquark baryon with hidden strangeness |qqqsˉs>. We also present new results on the narrow threshold structure X(1810) with M= 1807 ± 7 MeV and γ= 62 ± 19 MeV which is produced in the region of very small P ² _T < 0.01 GeV². The possibility of the Coulomb production mechanism for X(1810) is discussed. Received: 28 April 1999     

Calculation of spectroscopic constants and radiative parameters for A1Σ u+ -X1Σ g+ and B1Πu-X1Σ g+ electronic transitions of sodium dimer

Vibrational, rotational, and centrifugal spectroscopic constants and radiative parameters, i.e., the Einstein coefficients, oscillator strengths, and wave numbers for vibrational transitions in electronic systems of bands A ¹Σ _u ⁺-X ¹Σ _g ⁺ (0 ≤ v′ ≤ 25; 0 ≤ v″ ≤ 44), B ¹Π _u -X ¹Σ _g ⁺ (0 ≤ v′ ≤ 29; 0 ≤ v″ ≤ 47), and the radiative lifetimes for the vibrational levels of excited states of the sodium dimer, are calculated. The calculations are carried out based on semiempirical potential curves constructed in this study. The calculated spectroscopic constants and radiative lifetimes are compared to the experimental values.