The rotation of molecules in the gas phase can be coherently excited by irradiation with strong nonresonant short laser pulses, interacting with the molecular anisotropic polarisability. Such coherent rotational excitation has been attracting much attention because of the intriguing nature of the rotational wave packet thus created, and its wide applicability to dynamical studies and advanced optics. In this review article, we first survey various experimental schemes adopted so far for externally controlling molecular rotation, and then describe a new approach based on a quantum-state resolved spectroscopic probe for investigating coherent rotational excitation by intense nonresonant laser fields. Representative examples are given to show how the method provides detailed information on excitation pathways in wave-packet creation, and how it realises full quantum-state reconstruction of the rotational wave packet in a favourable case. We also describe an advanced wave-packet control, i.e. the creation and...

Coherent rotational excitation by intense nonresonant laser fields

Molecular interactions are best probed by scattering experiments. Interpretation of these studies has been limited by lack of control over the quantum states of the incoming collision partners. We report here the rotationally inelastic collisions of quantum-state prepared deuterium hydride (HD) with H2 and D2 using a method that provides an improved control over the input states. HD was coexpanded with its partner in a single supersonic beam, which reduced the collision temperature to 0–5 K, and thereby restricted the involved incoming partial waves to s and p. By preparing HD with its bond axis preferentially aligned parallel and perpendicular to the relative velocity of the colliding partners, we observed that the rotational relaxation of HD depends strongly on the initial bond-axis orientation. We developed a partial-wave analysis that conclusively demonstrates that the scattering mechanism involves the exchange of internal angular momentum between the colliding partners. The striking differences between H2/HD and D2/HD scattering suggest the presence of anisotropically sensitive resonances. Scattering of molecules at low temperature that are prepared in single quantum states illuminates the mechanism of rotationally inelastic collisions and reveals the reorientation of partner molecules. By correlating each outgoing partial wave with the incoming waves, partial-wave analysis of the scattering angular distribution determines the dominant short- and long-range anisotropies of the interaction potential.

Cold quantum-controlled rotationally inelastic scattering of HD with H 2 and D 2 reveals collisional partner reorientation

Following the H + H2 and F + H2 reactions, the fluorine atom – water system has the potential to become one of the best understood chemical reactions. Stationary points for the F + H2O potential energy surface have been located with the “Gold Standard” CCSD(T) method using the Dunning correlation consistent basis sets through quintuple zeta. The CCSD(T)/cc-pV5Z barrier height is prediced to be 2.5 kcal mol−1, less than previous estimates of 4–7 kcal mol−1. From higher level theoretical studies of the prototypical F + H2 reaction, this barrier should be less than 0.5 kcal mol−1 above the exact, nonrelativistic classical barrier height. 41 of the 49 DFT methods applied to F + H2O predict no barrier at all. The eight DFT methods that do predict a barrier show exothermicities that are somewhat too small. The CCSD(T)/cc-pV5Z entrance complex is bound by 3.4 kcal mol−1 relative to separated F + H2O. The analogous exit complex is bound by 5.9 kcal mol−1 relative to separated HF + OH.

The entrance complex, transition state, and exit complex for the F + H2O → HF + OH reaction. Definitive predictions. Comparison with popular density functional methods.

Modern computational methods have become so powerful for predicting the outcome for the H + H2 → H2 + H bimolecular exchange reaction that it might seem further experiments are not needed. Nevertheless, experiments have led the way to cause theorists to look more deeply into this simplest of all chemical reactions. The findings are less simple.

https://www.pnas.org/content/pnas/111/1/15.full.pdf

Is the simplest chemical reaction really so simple

We propose a method based on Stark-induced adiabatic Raman passage (SARP) for preparing vibrationally excited molecules with known orientation and alignment for future dynamical stereochemistry studies. This method utilizes the (J, M)-state dependent dynamic Stark shifts of rovibrational levels induced by delayed but overlapping pump and Stokes pulses of unequal intensities. Under collision-free conditions, our calculations show that we can achieve complete population transfer to an excited vibrational level (v > 0) of the H2 molecule in its ground electronic state. Specifically, the H2 (v = 1, J = 2, M = 0) level can be prepared with complete population transfer from the (v = 0, J = 0, M = 0) level using the S(0) branch of the Raman transition with visible pump and Stoke laser pulses, each polarized parallel to the z axis (uniaxial π − π Raman pumping). Similarly, H2 (v = 1, J = 2, M = ±2) can be prepared using SARP with a left circularly polarized pump and a right circularly (or vice versa) polarized S...

/pdf/stark-induced-adiabatic-raman-passage-for-preparing-2csl0ypjrv.pdf

Stark-induced adiabatic Raman passage for preparing polarized molecules.

Stimulated Raman pumping has been used to prepare oriented and aligned samples of H(2)(nu=1,J=1,2,3) and HD(nu=1,J=2) under collision-free conditions using the (1,0) S(0), S(1), Q(1), Q(2), and O(3) lines. The M-sublevel anisotropies were interrogated by polarized [2+1] resonance-enhanced multiphoton ionization via the (0,1) O(2), O(3), and S(1) lines of the E,F (1)Sigma(g) (+)-X (1)Sigma(g) (+) system. The optical excitation schemes employed in this study generate highly oriented and aligned molecular ensembles. We show that the H(2)(nu=1,J=2,M=0) and H(2)(nu=1,J=2,M=2) samples retain their initial polarization for greater than 100 ns and are therefore suitable candidates for targets or projectiles in future scattering experiments.

/pdf/preparation-of-oriented-and-aligned-h-2-and-hd-by-stimulated-5gllwi4xnm.pdf

Preparation of oriented and aligned H(2) and HD by stimulated Raman pumping.

An aligned sample of HD(v = 1, J = 2, MJ = 0) molecules is prepared under collision-free conditions using the S(0) stimulated Raman pumping transition. Subsequent coupling to the spins of the deuteronID and the proton IH causes the initial degree of alignment to oscillate and decrease as monitored over the time range from 0–13 μs via the O(2) line of the [2 + 1] REMPIE,F1Σ+g−X1Σ+g (0,1) band. The time dependence of the rotational alignment is also calculated using both a hierarchical coupling scheme in which the rotational angular momentum J is regarded first to couple to ID, and then the resultant Fi to couple to IH, to form the total angular momentum F and a non-hierarchical coupling scheme in which the HD energy level structure is not assumed to be diagonal in the |IH(JID)FiFMF> basis set. The experimental data is in good agreement with the non-hierarchical calculation but not with the hierarchical calculation, as expected for this system. Additionally, we calculate the time dependence of the H and D nuclear spin polarizations.

/pdf/time-dependent-depolarization-of-aligned-hd-molecules-24ahaz5gxi.pdf

Time-dependent depolarization of aligned HD molecules

Deuterium bromide (DBr) is expanded from a pulsed jet into a vacuum and a synchronized pulsed laser causes photodissociation of some of the DBr molecules to produce primarily (∼85%) ground-state bromine atoms (P23/2) and fast D atoms. The latter collide with the cold DBr molecules and react to produce molecular deuterium (D2) via two possible channels, the adiabatic channel D2+Br(P23/2) and the nonadiabatic channel D2+Br∗(P21/2), which are asymptotically separated in energy by the spin-orbit splitting (0.457 eV) of the bromine atom. Ion images are recorded for D2(v′=1, J′=16, 18–21), D2(v′=2, J′=6,7, 10–12, 14–16), and D2(v′=3, J′=2–5) for various collision energies. For the nonadiabatic production of spin-orbit-excited Br∗ in the D+DBr reaction for the conditions studied we estimate that this channel contributes 1% or less.

Search for Br∗ production in the D+DBr reaction

We have measured differential cross sections (DCSs) for the reaction H + D2 → HD(v′ = 2,j′ = 0,3,6,9) + D at center-of-mass collision energies Ecoll of 1.25, 1.61, and 1.97 eV using the photoloc technique. The DCSs show a strong dependence on the product rotational quantum number. For the HD(v′ = 2,j′ = 0) product, the DCS is bimodal but becomes oscillatory as the collision energy is increased. For the other product states, they are dominated by a single peak, which shifts from back to sideward scattering as j′ increases, and they are in general less sensitive to changes in the collision energy. The experimental results are compared to quantum mechanical calculations and show good, but not fully quantitative agreement.

Differential cross sections for H + D2 → HD(v′ = 2, j′ = 0,3,6,9) + D at center-of-mass collision energies of 1.25, 1.61, and 1.97 eV

Molecular deuterium is prepared in the J = 2, M = 0 sublevel of ν = 1 by stimulated Raman pumping of the ν = 0 S(0) line. Following optical excitation, the degree of alignment of the rotational angular momentum J oscillates in time caused by the coupling of J to the total nuclear spin angular momentum IT. This coupling is of two kinds, the interaction of J with the magnetic moments and the quadrupole fields of the two I = 1 deuterium nuclei. The alignment is monitored via the O(2) line of the E,F1Σg+ − X1Σg+ (0,1) band using [2+1] resonance enhanced multiphoton ionization for pump–probe delays from 0 to 20 μs. Using the hyperfine coupling constants found previously for the ν = 0 state (R. F. Code and N. F. Ramsey, Phys. Rev. A, 1971, 4, 1945), we are able to fit the time dependence essentially within our experimental error, but this requires that the presence of both IT = 0 and IT = 2 nuclear spin states for this o-deuterium level is properly weighted and taken into account.

Nate C.-M. Bartlett

Papers

Preparation of oriented and aligned H(2) and HD by stimulated Raman pumping.

Time-dependent depolarization of aligned HD molecules

Search for Br∗ production in the D+DBr reaction

Differential cross sections for H + D2 → HD(v′ = 2, j′ = 0,3,6,9) + D at center-of-mass collision energies of 1.25, 1.61, and 1.97 eV

Time-dependent depolarization of aligned D2 caused by hyperfine coupling