Lowest Vibrational States Research Articles

Rotational state selectivity in N2+X2∑g+(ν+= 0) by delayed pulsed field ionization spectroscopy via the a″1∑g+(ν′ = 0) state

The pulsed field ionization (PFI) spectrum of nitrogen is recorded involving overall transitions from the neutral ground state, X 1∑g + (ν″ = 0), to the lowest vibrational state of the ion, N2 + X 2∑g + (ν+ = 0). In our apparatus, designed for the study of state selected ion-molecule reactions, it is demonstrated that clean populations of ions with rotational angular momentum quantum number N + = 0-6 can be produced by the PFI of high-n Rydberg states. These Rydberg states are populated here in a (2 + 1′) two-colour excitation scheme, resonant at the two-photon level with the a″ 1∑g + (ν′ = 0) Rydberg state. More than 102 state selected ions are produced per laser pulse. Conclusive evidence is presented for rotational channel interactions being responsible for the enhanced relative intensities of the branches corresponding to transitions involving N + - J′ < 0 in the final step. Several interloper states contributing to this process are identified unambiguously. Detailed experiments on one threshold (N + ...

Calculations of magnetic hyperfine structure constants for the low-lying rovibrational levels of LiH, HF, CH+, and BH

Abstract Spin-rotation constants for the six lowest vibrational states and the first excited end-over-end rotational state were calculated for several isotopomers of the X 1 Σ + diatomic molecules: LiH, HF, CH + and BH. For the ground vibrational state, the rotational dependence of the spin-rotation constant was also investigated and was found to be very weak. The electronic calculations were done at various levels of the polarization propagator approximation using atomic basis sets adapted for calculations of magnetic properties. The calculated spin-rotation constants for the isotopomers of LiH and HF, namely their rovibrational dependence, are in good agreement with the available experimental data. The extremely large values of the spin-rotation constants of C and B in CH + and BH previously predicted by coupled perturbed Hartree-Fock calculations at the equilibrium internuclear distances were confirm by the present correlated and rovibrational averaged calculations. For the ground vibrational and first rotational state the spin-rotation constant of B in 11 BH in 11 CH + are predicted to be −459.70 and −1186.15 kHz, respectively. The results are discussed in the light of recent calculations of the nuclear quadrupole coupling constants, since the relative strengths of both the magnetic and electric parameters determined the resulting pattern of the hyperfine structure of molecular rovibrational spectra. Improved experimental values for the nuclear magnetic shielding constants of HF ( σ F = 418.0 ± 2.0, σ H = 28.6 ± 2.0) at the equilibrium geometry were derived from recent measurements and calculated diamagnetic contributions.

Optical spectra of metal dimers and trimers in superfluid helium.

Electronic spectra of ${\mathrm{Ca}}_{2}$, ${\mathrm{Cu}}_{2}$, and ${\mathrm{Ag}}_{3}$ have been studied in superfluid helium. The \ensuremath{\Sigma}-\ensuremath{\Sigma} transitions of ${\mathrm{Ca}}_{2}$ and ${\mathrm{Cu}}_{2}$ show vibrationally resolved absorption and emission bands close to those in free space. The emission has been observed only from the lowest vibrational states, reflecting rapid relaxation of the dimer vibration. Vibrational frequencies of the ground electronic states are slightly affected by the liquid. Anomalous spacings between the v\ensuremath{'}=0\ensuremath{\leftarrow}v\ensuremath{'}\ensuremath{'}=0 and v\ensuremath{'}=1\ensuremath{\leftarrow}v\ensuremath{'}\ensuremath{'}=0 absorption bands are discussed in terms of the coupling of dimer vibration and bubble bath modes.

Rotation-vibrational states of D3+computed using hyperspherical harmonics

Using the RKJK potential surface highly excited rovibrational states of D2H+ were calculated and characterized. Band origins are reported for energies up to 9300 cm-1. Term values of the 11 lowest vibrational states are presented with J upto J = 10 for the vibrational states (v 1,v 2,v 3) = (0,0,0) and (1,0,0),up to J = 8 for (0,1,0) and (0,0,1), up to J = 7 or J = 6 for (0,2,0), (2,0,0) and (0,3,0), and up to J = 4 for the states (0,0,2), (0,1,1), (1,1,0) and (1,0,1). Calculated transition frequencies for the ν1, ν2 and ν3 fundamental bands are compared with experimental data.

Microwave spectrum and conformation of mercaptoacetaldehyde

The microwave spectrum of the pyrolyzed product of 2,5-dihydroxy-1,4-dithiane has been observed. Analysis of the spectrum showed that the pyrolyzed product was mercaptoacetaldehyde. The conformation of this molecule was of a twisted skeleton structure. Strong satellite spectra were observed and the frequency of the lowest vibrational state was estimated to be 75 cm −1.

State-to-state rate constants for relaxation of highly vibrationally excited O2 and implications for its atmospheric fate

Collisional relaxation of highly vibrationally excited O2(X3Σg–, v) has been investigated for vibrational states from v= 18 to 27, using O2 and N2 as quenching partners. These excited states of O2 are now known to be produced in stratospheric ozone photolysis and this study gives information on their eventual atmospheric fate. State-to-state relaxation rate constants were obtained using the stimulated emission pumping method in the pump–dump–probe geometry. For self-relaxation, O2(v= 19) was found to decay in single vibrational-quantum steps and the rate constants were found to agree well with theoretical predictions which rely on an ab initio potential-energy surface. For vibrational levels above v= 25, a dark channel is observed, where molecules prepared in a state v do not appear in vibrational states v– 1 or v– 2. The most likely explanation to this dark channel is the onset of the chemical reaction O2(X 3Σg–, v 26)+ O2(X 3Σg–)→ O3+ O(3P)(1). The temperature-dependent data presented in this paper allow the derivation of activation energies for the dark channel when the reactants are prepared in vibrational states just below the known activation barrier of reaction (1). For these states, a fraction of the molecules ‘trickle down’ into lower vibrational states, allowing their kinetics to be probed. The derived activation energies are quantitatively consistent with the assignment of the dark channel to reaction (1). The first vibrational state which is above the activation barrier to reaction (1), v= 28, is not observed to ‘trickle down’, suggesting that it only follows the dark channel, i.e. reaction (1). The measurements diverge from the theory successful at v= 19 just at the energetic onset of reaction (1). Relaxation rate constants for N2 quench gas decrease with O2 vibrational quantum number and theory is not successful in reproducing these results. It appears that a more accurate O2–N2 potential-energy surface is needed.

Optothermal detection of nonradiative relaxation channels in electronically excited molecules

Optothermal detection has been used to observe nonradiative relaxation channels in aniline, p-bromoaniline, and trans-stilbene. p-Bromoaniline has no detectable fluorescence due to a heavy atom effect which increases the rate of intersystem crossing to the triplet state. An optothermal spectrum of p-bromoaniline was observed with the origin at 32 625 cm−1. For trans-stilbene, the differences between the laser excitation spectrum and the optothermal spectrum of the S1 state clearly show the onset of isomerization at ∼1250 cm−1 above the origin. Absolute quantum yields of fluorescence, Franck–Condon factors, nonradiative rates, and radiative rates have been obtained for a series of vibronic transitions. For low energy vibrational states, there is good agreement between the current study and previous work. For vibrational energies above the barrier of isomerization, predicted quantum yields do not agree with our experimental results.

Vibrational states of LiCN calculated from an ab initio potential energy surface

A high-level ab initio potential energy surface has been calculated for the LiCN molecule. Two linear and one T-shaped minimum energy structures have been found. The linear isocyanide structure of LiCN is the global minimum. The energies of the lowest vibrational states have been predicted and their interaction with the rotational motion has been studied. The molecule is extremely non-rigid.

Nascent state-resolved ClO in reactions and photodissociation processes

We report a detection technique for the complete state-resolved observation of nascent ClO generated in the photodissociation of parent molecules or formed as product in chemical reactions. The sensitivity and resolution of the applied (2+1) laser-induced fluorescence technique is demonstrated in the photodissociation of ClO 2 at 337-360 nm and in the important atmospheric reaction of Cl with O 3. In the photodissociation of ClO 2, ClO is observed in lower vibrational states ( v=0, 1, 2). the rotational temperature is found to be 650±60 K ( v=0). ClO from the reaction of Cl with O 3 is generated mainly in higher vibrational states ( v=3, 4).

Effective rotational constants for the three lowest vibrational states of ketene

This paper deals with the least-squares determination of effective rotational constants for the lowest three vibrational states of the ketene (CH2CO) molecule:v 5,v 6 andv 9. Several hundred rotational lines have been assigned in high-resolution Fourier-transform infrared spectra and were fitted to a standard, Watson Hamiltonian in theA-reduction formalism.

Rovibrational dependence of the ab initio nuclear quadrupole coupling constants in the X 3Σ − state of NH

The 14N and 2H quadrupole coupling constants of rovibrational levels of 14N 1H and 14N 2H in their ground electronic states have been calculated from molecular wavefunctions which explicity describe nuclear motion. For the lowest vibrational states the 14N quadrupole coupling is predicted to be relatively strong and to decrease rapidly with vibrational excitation. The deuteron coupling in 14N 2H is found to be weak and its behaviour is similar to that of other first-row hydrides. The change with rotational excitation is unimportant. For about six lowest vibrational levels, the quadrupole hyperfine patterns of 14N 2H in its X 3Σ − state are dominated by relatively strong nitrogen coupling.

Infrared diode laser spectroscopy of the ν3 fundamental and ν3+ν5−ν5 sequence bands of the C4 radical in a hollow cathode discharge

The infrared absorption spectrum of the linear C4 radical has been studied in an extension of the original observation of gas-phase C4 by Heath and Saykally [J. Chem. Phys. 94, 3271 (1991)]. The experiment was performed using a flowing mixture of acetylene and helium subjected to a hollow-cathode discharge, which was probed in the 1525–1570 cm−1 spectral region using a tunable diode laser spectrometer. Transitions with N-values up to 60 were measured. Their analysis yielded band origins, rotational, and centrifugal distortion parameters for the lower and upper vibrational states, and l-type doubling parameters for the degenerate bending states ν5 and ν3+ν5. In particular, the ν3 origin was determined to be 1548.6128(4) cm−1, the ground state rotational and centrifugal distortion parameters were B=4979.89(21) MHz and D=0.848(44) kHz, and the l-doubling parameters for ν5 was q5=10.98(13) MHz. This value for q5 was used to estimate the ν5 frequency of gas-phase C4 to be 160±4 cm−1. Both the l=0 and 2 components of the ν3+2ν5−2ν5 sequence band were also tentatively observed, but a detailed analysis was not yet possible. The results were completely consistent with a linear structure for the triplet ground state of C4, and showed no effects of quasilinearity such as that exhibited by C3.

The lifetimes of gas phase CO2˙− and N2O˙− calculated from the transition probability of the autodetachment process A− → A + e−

AbstractA procedure to calculate the quantum mechanical transition probability of a unimolecular primary chemical process, A− → A + e− is investigated for the circumstance where A− and A have different numbers of vibrational and rotational degrees of freedom (one is linear, the other not). A procedure is introduced to deal with the coupling between the vibrational and rotational motions. The proposed method was applied to calculating the lifetimes of CO2˙− and N2O˙− in the gas phase. The geometry optimizations and frequency calculations for CO2, CO2˙−, N2O, and N2O˙− are performed at HF, MP2, and QCISD(T) levels with 6‐31G* or 6–31+G* basis sets, in order to obtain reliable geometric and spectroscopic information on these systems. Lifetimes are calculated for several of the lower vibrational–rotational states of the anions, as well as for the Boltzmann distribution of states at 298 K. The lifetime of the lowest vibrational–rotational state of CO2˙−, is 1.03 × 10−4 s, and of the lowest vibrational state with rotational levels weighted by Boltzmann distribution at 298 K, 1.50 × 10−4 s. These values are in good agreement with the experimental number, 9.0 ± 2.0 × 10−5 s, and support the experimental evidence that CO2˙− was formed in its ground vibrational level by the techniques used. The lifetime of CO2˙− calculated with Boltzmann distribution over its vibrational and rotational levels at 298 K, is 1.51 × 10−5 s. There are no direct measurements of the lifetime of N2O˙−, but it was estimated to be greater than 10−4 s from experimental evidence. The predicted lifetimes of N2O˙−, at its lowest vibrational–rotational state (0 K) and lowest vibrational state with rotational levels weighted by the Boltzmann distribution at 298 K, are 238 and 19.1 s, respectively. The lifetime of N2O˙− at thermal equilibrium at 298 K is 6.66 × 10−2 s, indicating that electron loss from the excited vibrational states of N2O˙− is significant. This study represents the first theoretical investigation of CO2˙− and N2O˙− lifetimes. © 1994 John Wiley &amp; Sons, Inc.

Laboratory evidence for a possible non‐LTE mechanism of stratospheric ozone formation

Experimental evidence is given that supports the possibility of a previously unknown non‐LTE mechanism for stratospheric ozone formation, which could have a significant impact on the stratospheric ozone budget even if the quantum yield for production of highly vibrationally excited O2 in reaction (1), (averaged over all wavelengths shorter than 243nm) were as low as 0.2%. Stimulated emission pumping enabled preparation of individual vibrational states of O2(X³Σg−,19≤v≤27) and laser induced fluorescence was used to follow the time evolution of the prepared states and thereby determine the vibrational‐state‐specific total‐removal rate‐constants for relaxation by O2 and N2 at 295K. Self‐relaxation shows a sharp threshold for enhanced relaxation near the energy of O2(X³Σg−, v=26) which is coincident with the energetic threshold for reaction (2). The magnitudes of the self‐relaxation rate constants for O2(X³Σg−, v=26 and 27) are quantitatively consistent with the kinetic parameters of reaction (2). Relaxation by N2, while important for lower O2 vibrational states, is shown to be about 10 and 200 times slower than self relaxation for v=26 and v=27, respectively. These are the first two vibrational states of O2 that could form O3 via reaction (2).

Aniline-CH4 S1 vibrational dynamics studied with picosecond photoelectron spectroscopy

Intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) are measured for a number of vibronic states in the S1 electronic state of the aniline-CH4 complex. The detailed dynamics are monitored using picosecond threshold photoelectron spectroscopy which is shown to be a sensitive probe of van der Waals molecule dynamics. For the lowest vibrational states accessed, the 6a10 and 6a10+24 cm−1 bands, both IVR and VP are observed and their rates are independently determined. At higher excess energy, IVR becomes the rate limiting step and the rate for VP cannot be measured independently. The results are interpreted using a serial dissociation mechanism with a simple kinetic model description. The rates of the reaction are modeled using standard Rice–Ramsperger–Kassel–Marcus (RRKM) theory which qualitatively predicts rates consistent with the experimental observations.

Evidence for non‐local‐thermodynamic‐equilibrium rotation in the OH nightglow

Improved ground‐based Fourier transform infrared spectroscopic measurements of the OH Meinel (M) Δν= 2 and 3 nightglow emissions revealed unexpectedly intense transitions from high rotational levels. The rotational development in the P branches of the OH M (3,1), (6,3), and (7,4) bands has been followed to the P1(N″) transitions with N″ = 10, 12, and 13, respectively. These measurements indicate that ∼10% of the OH(X, ν′ = 7) column rotational population is typically in rotational levels with N′ ≥7, in sharp contrast with the corresponding local thermodynamic equilibrium estimate of ∼1%. The excess high‐N′ population also persists in the lower vibrational states of OH(X), as evidenced by the readily detectable high‐P1(N″) transitions in the (3,1) and (6,3) bands and by the presence of observable R branch heads in the (4,2) and (5,3) bands. The R heads in the (4,2) and (5,3) bands form at moderately high N′ and are typically much more intense than expected on the assumption of complete rotational‐translational equilibration throughout the OH M source region. These new results provide strong evidence for incomplete R‐T equilibration of OH(X²Π,3≤ν′≤7,J′) on a column basis under typical nightglow conditions.

Relaxation dynamics in the B(1/2) and C(3/2) charge transfer states of XeF in solid Ar

Dispersed laser induced fluorescence, and time domain measurements using the optical Kerr effect are applied to study the relaxation dynamics of Xe+F− (B 2Σ1/2 and C 2Π3/2) charge transfer states in solid Ar. Very fast vibrational relaxation is observed in the C emitting site: excitation near v=20 leads to population of v=0 of the C state in 13(±2) ps. In the B emitting site, the lower vibrational states relax sequentially. Relaxation times of 800(±30) ps for 1→0 and 250(±30) ps for 2→1, are measured directly; and 150(±30) ps for 3→2 and &amp;lt;30 ps for 4→3 are estimated from spectral intensities. A new, much faster relaxation channel, which leads to B(v=1, and v=0) is open to states above v=3 in the B emitting site. This fast channel has a relaxation time of 7(±1) ps and must involve multiple internal conversions among the nested electronic states in the ionic manifold. Under intense pumping, the excited population relaxes by stimulated emission. Stimulated radiative relaxation rates larger than 1.5×1011 s−1 are observed for B(v=0).

Vibrational effects on cross sections for elastic scattering of X-rays and fast electrons by H 2O molecules

Waller-Hartree and first Born cross sections for the elastic scattering of X-rays and fast electrons by H 2O are calculated for 80 geometries of the H 2O molecule using self-consistent-field electronic wave functions of good quality. The cross sections are then averaged over the five lowest vibrational states with wave functions calculated using the discrete variable representation. The differences between the vibrationally averaged cross sections and their equilibrium counterparts are less than 1.3% and 3% for X-ray and electron scattering respectively. The effects are greater in states with a stretch excited than a bend.

A study of the large amplitude motions of indoline through microwave spectroscopy andab initiocalculations

We report the gas phase microwave observation and subsequent analysis of the ring puckering level splitting of indoline and its N-D isotopomer. The spectra of the four lowest vibrational states are modelled through a two-dimensional vibrational Hamiltonian to furnish a coherent picture. According to this new picture, five-membered rings with a single double bond and five-membered rings in which the double bond is fused with a benzenic ring share a similar low ring puckering barrier. Discrepancies with previous work are ascribed to the use of a one-dimensional vibrational Hamiltonian. Ab initio calculations concur with the present analysis.

Ab initio potential energy surface for IHI −b. Simulation of IHI − photodetachment spectra

Ab initio calculations are used to develop a potential energy surface for the IHI − molecule. Wavefunctions for some of the lowest vibrational states associated with this surface are then used to calculate transition state photodetachment spectra. The resulting spectra show small but important changes relative to those obtained previously with a harmonic force field. The spectra associated with IHI − vibrationally excited states are significantly different from the ground state, revealing more details of the IHI transition state structure.