Specific Rotational Levels Research Articles

Preparation of a selected high vibrational energy level of isolated molecules.

Stark induced adiabatic Raman passage (SARP) allows us to prepare an appreciable concentration of isolated molecules in a specific, high-lying vibrational level. The process has general applicability, and, as a demonstration, we transfer nearly 100 percent of the HD (v = 0, J = 0) in a supersonically expanded molecular beam of HD molecules to HD (v = 4, J = 0). This is achieved with a sequence of partially overlapping nanosecond pump (355 nm) and Stokes (680 nm) single-mode laser pulses of unequal intensities. By comparing our experimental data with our theoretical calculations, we are able to draw two important conclusions: (1) using SARP a large population (>1010 molecules per laser pulse) is prepared in the (v = 4, J = 0) level of HD and (2) the polarizability α00,40 (≅0.6 × 10-41 C m2 V-1) for the (v = 0, J = 0) to (v = 4, J = 0) Raman overtone transition is only about five times smaller than α00,10 for the (v = 0, J = 0) to (v = 1, J = 0) fundamental Raman transition. Moreover, the SARP process selects a specific rotational level in the vibrational manifold and can prepare one or a phased linear combination of magnetic sublevels (M states) within the selected vibrational-rotational level. This capability of preparing selected, highly excited vibrational levels of molecules under collision-free conditions opens new opportunities for fundamental scattering experiments.

Infrared spectroscopy of molecular ions in selected rotational and spin-orbit states.

First results are presented obtained with an experimental setup developed to record IR spectra of rotationally state-selected ions. The method we use is a state-selective version of a method developed by Schlemmer et al. [Int. J. Mass Spectrom. 185, 589 (1999); J. Chem. Phys. 117, 2068 (2002)] to record IR spectra of ions. Ions are produced in specific rotational levels using mass-analyzed-threshold-ionization spectroscopy. The state-selected ions generated by pulsed-field ionization of Rydberg states of high principal quantum number (n ≈ 200) are extracted toward an octupole ion guide containing a neutral target gas. Prior to entering the octupole, the ions are excited by an IR laser. The target gas is chosen so that only excited ions react to form product ions. These product ions are detected mass selectively as a function of the IR laser wavenumber. To illustrate this method, we present IR spectra of C2H2 (+) in selected rotational levels of the (2)Πu,3/2 and (2)Πu,1/2 spin-orbit components of the vibronic ground state.

Visible/Infrared Dissociation of NO3: Roaming in the Dark or Roaming on the Ground?

We present a DC slice imaging study of roaming dynamics in the photodissociation of the nitrate radical, NO3, contrasting pure visible excitation with a combination of visible and CO2 laser excitation at 10.6 μm. Images of specific rotational levels of NO are seen to reflect dissociation on the ground and first excited electronic states, as reported in previous work. The branching is obtained for specific rotational levels by comparison to quasiclassical trajectory calculations of the dynamics on these two surfaces. The results for the visible dissociation are found to be very similar to the combination of visible and infrared, raising questions about the nature of the coupling of these surfaces, the extent to which roaming takes place on both, and how the final product branching is determined.

State-specific detection of trapped HfF+ by photodissociation

We use (1+1′) resonance-enhanced multiphoton photodissociation (REMPD) to detect the population in individual rovibronic states of trapped HfF+ with a single-shot absolute efficiency of 18%, which is over 200 times better than that obtained with fluorescence detection. The first photon excites a specific rotational level to an intermediate vibronic band at 35000–36500cm−1, and the second photon, at 37594cm−1 (266nm), dissociates HfF+ into Hf+ and F. Mass-resolved time-of-flight ion detection then yields the number of state-selectively dissociated ions. Using this method, we observe rotational-state heating of trapped HfF+ ions from collisions with neutral Ar atoms. Furthermore, we measure the lifetime of the 3Δ1v=0,J=1 state to be 2.1(2)s. This state will be used for a search for a permanent electric dipole moment of the electron.

Velocity map imaging of the dynamics of bimolecular chemical reactions

The experimental technique of velocity map imaging (VMI) enables measurements to be made of the dynamics of chemical reactions that are providing unprecedented insights about reactive scattering. This perspective article illustrates how VMI, in combination with crossed-molecular beam, dual-beam or photo-initiated (Photoloc) methods, can reveal correlated information on the vibrational quantum states populated in the two products of a reaction, and the angular scattering of products (the differential cross section) formed in specific rotational and vibrational levels. Reactions studied by VMI techniques are being extended to those of polyatomic molecules or radicals, and of molecular ions. Subtle quantum-mechanical effects in bimolecular reactions can provide distinct signatures in the velocity map images, and are exemplified here by non-adiabatic dynamics on coupled potential energy surfaces, and by experimental evidence for scattering resonances.

Low-energy collisions with atomic and molecular ions in a photocathode electron target

Dielectronic recombination of highly charged atomic ions and dissociative recombination of molecular ions have been investigated using fast ion beams merged with cold intense electron beams (down to <1meV beam temperature) from a GaAs photocathode. For beryllium-like germanium ions (Ge28+) isolated fine structure Rydberg resonances (n = 9 and 14) were observed at <0.2eV revealing configuration interaction with triply excited resonances and appearing as candidates for deriving radiative shifts of the 2s2-2s2p(3P0) excitation energy. For hydrogen fluoride ions (HF+) dissociative recombination with the cold electrons, producing n = 2 hydrogen atoms, was seen to occur for specific excited initial rotational levels. These levels were identified by their fragment energies down to about 4meV and it is envisaged to measure these energies with sub-meV accuracy.

Tensor cross sections and collisional depolarization of OH(X 2Π) in collisions with helium

Tensor cross sections for collisions of OH(X (2)Pi) with helium are computed using accurate ab initio potential energy surfaces computed by Lee et al. We use these cross sections to predict rate constants for collisional depolarization of specific rotational fine-structure levels of OH(X (2)Pi) in collisions with helium. Good agreement is found with the results of the two-color polarization spectroscopy experiments of Paterson et al. [J. Chem. Phys. 129, 074304 (2008)]. By examination of partial cross sections, the range of impact parameters at which collisional depolarization occurs is shown to be similar to that for rotationally inelastic collisions.

Infrared and far-infrared spectroscopy of 13CH 3OH: TeraHertz laser lines and assignments

We use a 13CO 2 laser as optical pumping source to search for new THz laser lines generated from 13CH 3OH. Nineteen new THz laser lines (also identified as far-infrared, FIR) ranging from 42.3 μm (7.1 THz) to 717.7 μm (0.42 THz) are reported. They are characterized in wavelength, offset, relative polarization, relative intensity, and optimum working pressure. We have assigned eight laser lines to specific rotational energy levels in the excited state associated with the C–O stretching mode.

Vibrational excitation of CO molecules by infrared free electron laser

This paper describes the vibrational excitation of CO molecules by FEL-SUT (Free Electron Laser at Tokyo University of Science). FEL is tuned to 4.67 μm, corresponding to the X 1Σ +( v″=1)←( v″=0) transition of CO. The population of the vibrationally excited states is probed by the (2+1) resonance enhanced multiphoton ionization technique via the B 1Σ + Rydberg state. We find that ∼10% of the population in v″=0 is excited to v″=1. In spite of the existence of the resonant vibrational ladder climbing channels via specific rotational levels, no signal originating from v″=2 is detectable. The predissociation in the B 1Σ +( v′=2) state is believed to be responsible for the absence of CO + signals.

Blackbody-radiation-assisted laser cooling of molecular ions.

The translational motion of molecular ions can be effectively cooled sympathetically to temperatures below 100 mK in ion traps through Coulomb interactions with laser-cooled atomic ions. The distribution of internal rovibrational states, however, gets in thermal equilibrium with the typically much higher temperature of the environment within tens of seconds. We consider a concept for rotational cooling of such internally hot, but translationally cold, heteronuclear diatomic molecular ions. The scheme relies on a combination of optical pumping from a few specific rotational levels into a "dark state" with redistribution of rotational populations mediated by blackbody radiation.

Rotational level dependence of ground state recovery rates for OH X 2Π(v″=0) in atmospheric pressure flames using the picosecond saturating-pump degenerate four-wave mixing probe technique

We report the first direct measurement of the rotational level dependency of the rate of recovery of initially depleted levels in the electronic ground state X 2Π(v″=0) of OH produced in different flame environments at atmospheric pressure. The initial depopulation of a specific rotational level is accomplished by an intense picosecond pump pulse at 308 nm to partially saturate the electronic A 2Σ–X 2Π(0,0) transition. The recovery of the depleted ground state population then is monitored by probing the same level via the (1,0) band at 283 nm using picosecond degenerate four-wave mixing (DFWM). Both laser wavelengths were derived from the pulse-amplified and frequency doubled output of two independently tunable distributed feedback dye lasers operated with Rh101 and Rh6G in ethanol, respectively, and pumped with the second harmonic of a frequency doubled ps-Nd:YAG laser. It is shown that the rate of repopulation of the depleted ground state levels decreases by 54% and 50% with increasing rotational quantum number, N″, ranging from 2–16 and 2–13 for stoichiometric CH4/air and H2/O2/He flames, respectively. Within experimental error their absolute values in both flames are equal and are not noticeably sensitive to an unequal depletion of the Zeeman sublevels, as created for different polarization configurations of the saturating pump beam and the DFWM probe beams. The rate of (1.8±0.4)×109 s−1 averaged over all rotational transitions investigated is smaller by a factor of 3 than the corresponding average rate of the temporal DFWM signal intensity decay determined by us previously. The rate also is smaller than total depopulation rates obtained in the excited A 2Σ+ state of OH for similar flame conditions.

Cooling and trapping of molecules in highly excited rotational states

Collisional relaxation of highly rotating hydrogen molecules is investigated as a function of energy. Calculations demonstrate that inelastic collisions are dramatically suppressed for specific rotational levels of the molecule as the energy is lowered due to the closing of quasiresonant rotation-vibration channels. It is predicted that a ${}^{3}\mathrm{He}$ buffer gas may be used to load these highly excited molecules into a trap without a significant loss of population. It is further predicted that evaporative cooling may be used to cool the ``super rotors'' to even lower temperatures.

High-resolution pulsed-field ionization photoelectron–photoion coincidence spectroscopy:

Abstract We have established a high-resolution vacuum ultraviolet (VUV) photoionization–photoelectron facility at the chemical dynamics beamline of the advanced light source. This facility is equipped with a 6.65-m monochromator, which has demonstrated a resolving power of ≈100 000 in the range of 6–30 eV. This resolving power is close to that achieved in common VUV lasers. Taking advantage of this high-resolution capability, we have developed novel synchrotron based pulsed-field ionization–photoelectron (PFI–PE) schemes, attaining resolutions of 1–5 cm−1 (FWHM), which are also similar to that attained in laser-based PFI–PE studies. Most recently, we have introduced a generally applicable PFI–PE–photoion coincidence (PFI–PEPICO) method, making possible the internal state selection for cations to a resolution of 4–5 cm−1 (FWHM) limited only by the PFI–PE measurement. The PFI–PEPICO study of the reaction O2+hν→O+(4S)+O(3P)+e− demonstrates that the dissociation dynamics of diatomic cations can be examined in specific rotational levels using this method. This study also supports that the dissociation threshold for O+(4S)+O(3P) lies at O2+(b4Σg−, v+=4, N+=9). In the PFI–PE–PICO study of CH4 (C2H2), we show that the 0 K dissociative photoionization threshold for CH3+ (C2H+) from CH4 (C2H2) can be determined unambiguously by the disappearance energy of the CH4+ (C2H2+) parent ion to an accuracy of ±0.001 eV. These measurements, when combined with accurate ionization energies for CH3, CH4 and C2H2 obtained by PFI–PE measurements, have allowed the determination of highly accurate 0 K bond dissociation energies for H–CH3, H–CH3+, and H–C2H+. The PFI–PEPICO study of 2-C3H7X (X=Cl, Br and I) has yielded more accurate values for the heat of formation of C3H7+ and the proton affinity of C3H6.

Hydride abstraction by NO+ from ethanol: Effects of collision energy and ion rotational state

The effects of NO+ rotational state and collision energy on the reaction NO++C2H5OH→HNO+C2H4OH+ were studied in a guided-ion-beam instrument over the collision energy range from 50 meV to 3.7 eV. Integral cross sections for the reaction are presented. NO+ is prepared in specific rotational levels (N+=0,1 and N+=10) by means of mass-analyzed threshold ionization. Ab initio calculations were used to probe stationary points on the potential energy surface. The reaction is sharply inhibited by collision energy, suggesting a bottleneck for reaction. If rotational energy had a similar effect, ∼50% inhibition from N+=10 excitation would be observed at low collision energy. Instead, rotation is found to have no effect within experimental error. A precursor complex mechanism is proposed to explain the results.

Vibrational Energy Gain in the ν2 Bending Mode of Water via Collisions with Hot Pyrazine (Evib = 37900 cm-1): Insights into the Dynamics of Energy Flow

Energy gain into specific rotational levels of vibrationally excited H2O(010) resulting from collisions with highly vibrationally excited pyrazine in a low-pressure, 298 K environment was investigated using high-resolution transient infrared absorption spectroscopy of water at λ ≈ 2.7 μm. Vibrationally excited pyrazine with 37900 cm-1 internal energy was generated by 266 nm optical excitation to an electronically excited singlet state of pyrazine, followed by rapid radiationless decay to the ground electronic state. Collisions between highly excited pyrazine and water that result in excitation of the ν2 bending vibrational mode (ν2 = 1594 cm-1) in water were studied by monitoring the time-resolved appearance of individual rotational states of H2O(010). Transient absorption signals were obtained for a number of rotational states with Erot ≤ 811 cm-1. The nascent distribution of rotational states for the scattered, vibrationally excited water molecules, H2O(010), is well characterized by a Boltzmann rotatio...

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.

Product quantum-state-dependent anisotropies in photoinitiated unimolecular decomposition

Angular distributions of state-selected NO and O products in the photoinitiated unimolecular decomposition of jet-cooled NO2 have been measured by using both the photofragment ion imaging technique with velocity map imaging and ion time-of-flight translational spectroscopy. The recoil anisotropy parameter of the photofragments, β, depends strongly on the rotational angular momentum of the photoproduct. O(3Pj=2,0) angular distributions are recorded at photolysis wavelengths 371.7, 354.7, and 338.9 nm. At these wavelengths, respectively, vibrational levels v=0, v=0,1 and v=0–2 of NO are generated. In addition, β values for NO(v=2) in specific high rotational levels are determined at ∼338 nm. The experimental observations are rationalized with a classical model that takes into account the transverse recoil component mandated by angular momentum conservation. The model is general and applicable in cases where fragment angular momentum is large, i.e., a classical treatment is justified. It is applied here both to the experimental NO2 results, and results of quantum calculations of the vibrational predissociation of the Ne–ICl van der Waals complex. It is concluded that deviations from the limiting β values should be prominent in fast, barrierless unimolecular decomposition, and in certain dissociation processes where a large fraction of the available energy is deposited in rotational excitation of the diatom. The application of the model to NO2 dissociation suggests that the nuclear dynamics leading to dissociation involves a decrease in bending angle at short internuclear separations followed by a stretching motion. This interpretation is in accord with recent theoretical calculations.

Mechanism for the Production of6Li2and7Li2Ultracold Molecules

Calculations are presented of the spontaneous emission probabilities and oscillator strengths of transitions between the vibrational energy levels of the ground and excited singlet and triplet states of the molecules6Li2and7Li2. The fractions of transitions into the continuum and the discrete levels are estimated and it is shown that the translationally cold molecules can be produced during photoassociation experiments with high efficiency. The data are used to quantify a scheme by which translationally cold molecules can be created in specific rotational and vibrational levels.

Mechanism for the production of vibrationally excited ultracold molecules of 7Li 2

A scheme is described by which translationally cold molecules can be created in specific rotational and vibrational levels and it is suggested they be employed to test Wigner's threshold laws for inelastic collisions. Calculations are presented of the spontaneous emission probabilities and the absorption oscillator strengths of transitions between the vibrational energy levels of the ground and excited triplet states of the molecules 7Li 2. The fractions of transitions into the continuum and into the discrete levels are estimated and it is shown that the translationally cold molecules can be produced efficiently in photoassociation experiments. The photoabsorption rate coefficients are computed.