Lowest Rotational States Research Articles

Fragment recoil anisotropies in the photoinitiated decomposition of HNCO

The photofragment ion imaging technique is used to determine product recoil anisotropy parameters, β, and correlated state distributions in the S1(1A″)←S0(1A′) photoinitiated decomposition of HNCO into three competing channels: (1) N3H+CO, (2) H+NCO, and (3) N1H+CO [where NH3 and NH1 denote NH(X 3Σ−) and NH(a 1Δ), respectively]. In particular, the region in the vicinity of the N1H+CO threshold is investigated. The measured recoil anisotropies fall into two distinct groups corresponding to time scales of <1 ps (β<−0.6), and >5–10 ps (β≅0.0). With 230.1 nm photolysis, CO(J=0–14) originating in channel (3) is produced with β=−0.8±0.05 via direct dissociation on S1 above a barrier of 470±60 cm−1. CO at low J-states appears with most of the available energy in the translational degree of freedom and is correlated with NH1 in its lowest rotational states. A small contribution to channel (3) from S0 dissociation (observed mainly for J=14,15) gives rise to an isotropic recoil distribution, and a hotter correlated NH1 rotational distribution. At the same wavelength, CO correlated with NH3 is identified by its high translational energy and exhibits an isotropic angular distribution. We propose that the pathway leading to its formation is S1→S0→T1. H-atom signals from channel (2) have isotropic angular distributions at photolysis wavelengths 243−215 nm; this places a lower limit of 8140 cm−1 on the barrier to direct dissociation on S1 to channel (2). The >5 ps time scale for the appearance of channel (2) implies dissociation on S0 following internal conversion. The mechanism described here for the one-photon decomposition of HNCO in the wavelength region 260-230 nm is in accord with other available experimental and theoretical findings.

Ortho-para separation of molecules using optical orientation of atoms in hyperfine Zeeman states

Ortho-para separations of ${}^{7}{\mathrm{Li}}_{2}$ and ${}^{6}{\mathrm{Li}}_{2}$ molecules were achieved by using a monochromatic laser beam to align atoms into a single nuclear spin state. This approach is unlike the cryogenic cooling technique, which has been successful only on very light molecules (such as ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$), which have large energy gaps between the lowest rotational states. The atoms were ``pumped into'' a single hyperfine Zeeman state with defined electron and nuclear spin orientations. The well-resolved rotational structures of the lithium molecules, as shown by laser-induced fluorescence spectra, revealed the enrichment of ortho-components over para-components. Using laser radiation with a power density of $\ensuremath{\sim}10{\mathrm{m}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ can increase the amount of ortho-components by a factor of 2 for ${}^{7}{\mathrm{Li}}_{2}$ vapor and by a factor of 3 for ${}^{6}{\mathrm{Li}}_{2}$ vapor on a time scale of milliseconds. This paper proposes a model to account for the experimental observations. These increases in the concentrations of ortho-components to para-components are explained by a combined effect of (i) a decrease in molecular density due to the electron spin orientation and (ii) a transfer of nuclear spin momenta from oriented atoms to molecules. The exchange reaction, ${\mathrm{Li}}^{\ensuremath{'}}{+\mathrm{L}\mathrm{i}}_{2}{\ensuremath{\rightarrow}\mathrm{Li}}_{3}{\ensuremath{\rightarrow}\mathrm{Li}}^{\ensuremath{'}}\mathrm{L}\mathrm{i}+\mathrm{L}\mathrm{i},$ appears to transfer the spin orientation from atoms to molecules. A substantial number of ortho-para separations can be achieved through multiple exchange collisions. Applications of this technique to other homonuclear diatomic molecules and polyatomic molecules with nuclear exchange symmetry are predicted.

Three Dimensional Wavepacket Simulation on the H Atom Scattering for the Full Reaction of CF3H + Ar (3P) → CF3* + H + Ar

AbstractThree dimensional (3D) wavepacket simulation on the H atom scattering is carried out for a full reaction of CF3H + Ar(3P) → CF3* + H + Ar for the H‐end orientation since our previous study on the 2D wavepacket simulation showed a quantum interference effect only for collisions at the H‐end of CF3H. We treat the present 3D wavepacket simulation in a time‐dependent SCF (TDSCF) scheme, which enables us to reduce 3D to a coupled two and one dimensional (2D+1D) problem. The initial wavepacket of CF3H is prepared in the lowest rotational state, i.e., |JKM > = |111>. The calculated snapshots of the scattered wavepacket for the Fiend orientation show a clear quantum interference effect again in the angular distribution of the product H atom due to the so‐called one‐atom cage effect on the heavy‐light‐heavy collision configuration. Thus the present simulation suggests that a continuous wave character of the wavepacket would be responsible for such a relatively clear interference effect in the full reaction, unlike a blurred interference effect with a pulse‐type of wavepacket excitation in the half reaction.

State-to-State, Rotational Energy-Transfer Dynamics in Crossed Supersonic Jets: A High-Resolution IR Absorption Method

A high-resolution IR absorption method is presented for the experimental determination of state-to-state, integral and differential cross sections for rotationally inelastic energy transfer. An infrared chromophore, cooled into its lowest rotational state(s) in a pulsed supersonic expansion, is rotationally excited with low collision probability by a gas pulse from a second supersonic jet. The initial and final populations of the infrared absorber are monitored as a function of J state and of Doppler detuning, via direct absorption of narrow bandwidth light from a continuously tunable, CW infrared laser. The scattered and unscattered species are detected with Doppler-limited spectral resolution (≲0.01 cm-1), providing quantum-state selectivity not attainable with time-of-flight energy-loss methods. The infrared-based probe also permits study of a much wider class of absorbing species inaccessible to ultraviolet/visible laser-induced fluorescence (LIF) or resonance-enhanced multiphoton ionization (REMPI) m...

Effect of hyperfine depolarization upon creation and detection of alignment in free-jet expansions via selective photodissociation

The creation of alignment by photodissociation is a well-accepted process. When an isotropic sample of gas is subjected to a strong linearly polarized laser pulse at a frequency at which the photodissociation cross section is large, the surviving molecules are usually aligned. If the transition is parallel, μ lies along the internuclear axis (ΔΛ=0) and the surviving molecules will be peaked around M=0, while for a perpendicular transition (ΔΛ=1) the surviving molecules will be peak around M=J. Although this effect has been seen in laser cavities and in the focus of laser beams, it has not been used to create aligned pulses of gas from free-jet expansions. We present the theoretical calculations for the practical creation of alignment in short free-jet gas pulses via saturation photodissociation. Our methodology allows the propagation of the laser light along any direction and with any polarization, the quantification of the effect of hyperfine and electron spin depolarization upon the creation of alignment, direct comparison of the degree of alignment created in parallel versus perpendicular transitions when the polarizations are set to cause M=J versus M=0 peaked distributions, experimental determination of the degree of alignment after depolarization using a second fixed frequency laser, and experimental determination of the degree of alignment prior to depolarization without the use of an additional laser. Our calculations show that hyperfine and electron spin depolarization are the limiting forces in the creation of aligned pulses of gas. These effects are most pernicious in free-jet expansions where only the lowest rotational states are populated and therefore even modest values of nuclear spin and electron spin can effect large depolarizations. The calculations show that these depolarization effects can be effectively mitigated by three methods: (1) using molecules with small Be values, (2) limiting the free-jet expansion so the rotational temperature is above about 50 K, and (3) employing parallel photodissociation transitions (ΔΛ=0). For very cold expansions, parallel transitions are less susceptible to depolarization than perpendicular transitions (ΔΛ=1) because in parallel transitions, the low J states are selectively photodissociated.

New vibrational bands of CH 2 ( [formula omitted

Singlet methylene has been produced in its lowest rotational states by photolysis of jet-cooled ketene at photolysis wavelengths near threshold, and the laser-induced fluorescence excitation spectrum has been recorded from 16 170 to 18 250 cm −1. The spectral simplification obtained has allowed identification of several previously unknown vibrational bands in the upper state. Three of the new bands are assigned as combination bands involving the ν 1 vibration of the b ̃ state; they should allow a more accutate determination of this mode's frequency and anharmonic constants. The variations in intensity with photolysis wavelength and the photolysis energy thresholds determined by photofragment excitation spectra for selected transitions permit unambiguous assignments of the lower states. The techniques presented here provide important tools for dealing with this congested and highly perturbed spectrum.

Supercooled molecules in molecular beams of ICl and IBr

The properties of molecular beams of ICl and IBr built by means of the seeded beam technique were investigated experimentally. The axial velocity distribution was measured in a pseudorandom time-of-flight experiment and the rotational state distribution was estimated from the focusing properties of the beam in an alternate gradient focusing system. From a comparison between the computer simulation of a focusing experiment and actual measured results, it can be stated that the population of the very lowest rotational states with rotational quantum numbers J=0, 1, 2 do not deviate significantly from a Boltzmann distribution. The rotational temperature is not very different from the translational temperature and has a value between 1 and 3 K. Collision numbers which are a measure of the translational rotational relaxation, are estimated from the experimental results.

Raman Scattering Spectra of Methane in Solid Rare Gases

Raman scattering spectra of ν 1 and ν 3 vibrational modes of methane have been observed for CH 4 and CD 4 in solid Kr, and for CH 4 in solid Xe. Fine structures due to the vibration-rotation states have been resolved in the spectrum of the ν 3 mode. The observed fine structures in the spectrum of CH 4 in Kr are examined to be in a good agreement with the theory of Kobashi, Kataoka and Yamamoto. The prominent doublet structures in this spectrum are due to the S-branch ( Δ J=2) type transitions from the lowest rotational state of the A-methane.

Nanosecond spectroscopy: kinetics of state selected species

OH is chemically produced by photolysis of ozone in a O 3/H 2O mixture with a nanosecond uv laser pulse. The OH-radicals were observed immediately after their formation nearly collision free by measuring the absorption intensities of the X 2¶ 3 2 (v'' = O, K'') → A 2σ +(v' = O, K'') transitions. The scanning of the rotational spectrum is performed by pressure tuning of the dye laser. The population of the Λ - doublets can be described by two different rotational temperatures. In the four lowest rotational states we observe inversion and in the higher rotational states antiinversion in the Λ - doublets. For K''⩾ 7 there is a strong deviation from these thermal distributions.

NMR studies of single crystals of H2: II. The spectrum of isolated ortho-H2 pairs

The NMR spectrum of isolated ortho-H2 pairs has been measured on two single crystals of H2 with ortho concentrations below about 2%. The experiments were carried out over a temperature range between 0.025 and 1 K, and spectra were recorded with the magnetic field applied in a number of directions. Extending the previous analysis of the spectrum for isolated ortho-H2 to the present data led to the determination of the crystal orientation. The complicated pair spectrum includes the signals from the nine nonequivalent pair orientations in the crystal and is a function of the two lowest rotational states. These states are split by an energy δ, which is different for the pairs with axis respectively in and out of the basal plane of the crystal (denoted IP and OP), namely % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaacqqHuoarcaGGOa% Gaam4taiaadcfacaGGPaGaai4laiaadUgadaWgaaWcbaGaamOqaaqa% baGccqWIdjYocaaI2aGaaGinaiaad2gacaWGlbaaaa!3DD4!\[\Delta (OP)/k_B \simeq 64mK\] and % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaacqqHuoarcaGGOa% GaamysaiaadcfacaGGPaGaai4laiaadUgadaWgaaWcbaGaamOqaaqa% baGccqGH8aapcaaI1aGaaeiiaiaad2gacaWGlbaaaa!3D85!\[\Delta (IP)/k_B < 5{\rm{ }}mK\].

Methyl group tunneling rotation in the lowest nπ* state of toluquinone. An optically detected ENDOR, LAC and CR study

In this paper we report and discuss the effects of methyl group tunneling rotation on the methyl proton ENDOR, LAC and CR spectra in the lowest triplet state of toluquinone at 1.8 K. From a detailed analysis of the ENDOR spectra in the lowest rotational state (A) we obtain for the methyl protons the following isotropic hyperfine interaction constants: A 1 = A 2 = 34.4 MHz and A 3 = −53.7 MHz. The tunneling frequency (3 J) is calculated to be 2.9 GHz. The most likely equilibrium configuration of the torsional oscillator is found to be the one whereby the molecular plane is a mirror plane and the out-of-plane methyl protons point towards the closest oxygen atom. We also show that cross-relaxation between |A> and |E> states of translational equivalent toluquinone molecules is responsible for the observed sidebands in the level-anti-crossing spectra of toluquinone.

Photodissociative formation of ion pairs from molecular hydrogen and the electron affinity of the hydrogen atom

Photodissociative production of ion pairs from H 2 has been observed in the wavelength range 706–718 A at spectral resolution of 0.4 and 0.22 A. From measured thresholds for production of H − from H 2 molecules in each of the three lowest rotational states, the lower bound EA(H) ⩾ 0.754 ± 0.002 eV is obtained, in excellent agreement with the theoretical electron affinity of 0.75421 eV. For formation of D − from D 2, a threshold assigned to molecules in the rotational state J = 2 has been measured, from which the bound EA(D) ⩾ 0.757 ± 0.005 eV is obtained. Negative ion yield curves are presented for hydrogen.

A Quantum Model for Bending Vibrations and Thermodynamic Properties of C3

A quadratically perturbed square well potential is used to derive quantized bending mode vibrational energy levels for the C3 molecule. Coupling with rotational modes and l doubling is neglected for simplicity. The model is constrained to a best fit with observed lower vibrational levels in the lowest rotational state, and subject to this constraint the upper vibrational levels have maximum possible divergence. Thus a lower limit for the partition function and the entropy of C3 is established; the neglected rotational coupling has little influence on these quantities because the splitting of levels is almost symmetrical. The limits obtained support the classical model of Strauss and Thiele for the thermodynamic properties of C3 rather than the estimates listed in current JANAF Thermochemical Tables, and imply that recent graphite vapor pressure measurements made by Zavitsanos and by Wachi and Gilmartin are more correct than earlier measurements.

Magnetic moments and magnetic dipole transitions in deformed nuclei

See also preceding abstract. The magnetic moments of the lowest rotational states of even-even nuclei and the probabilities of magnetic dipole transitions between these states are calculated on the basis of the model suggested by Krutov in 1968 the non-axiality of the nuclear equilibrium shape and the difference between the mass and charge distributions being taken into account. The results of calculations are compared with the available experimental results, the agreement being reasonably satisfactory.

Inelastic Scattering of 4-Mev Protons and Deuterons from Separated Isotopes ofDy161,Dy163,Yb171, andYb173

The inelastic scattering of protons and deuterons from the odd isotopes of Dy and Yb has been measured at energies around 4 Mev in a heavy-particle spectrometer at a scattering angle of 145\ifmmode^\circ\else\textdegree\fi{}. Isotopically pure targets were prepared in an electromagnetic isotope separator. Inelastic groups corresponding to the two lowest rotational states were observed in ${\mathrm{Dy}}^{161}$ at 44 kev and 102 kev, in ${\mathrm{Dy}}^{163}$ at 74 kev and 170 kev, in ${\mathrm{Yb}}^{171}$ at 67 kev and 76 kev, and in ${\mathrm{Yb}}^{173}$ at 78 kev and 181 kev. The reduced electric quadrupole transition probabilities for these levels were determined.