The dynamics of "stretched molecules": experimental studies of highly vibrationally excited molecules with stimulated emission pumping

Cold collisions of hot molecules

In this Perspective, we review our recent work on rotationally inelastic collisions of highly vibrationally excited NO molecules prepared in single rotational and parity levels at v = 10 using stimulated emission pumping (SEP). This state preparation is employed in a recently developed crossed molecular beam apparatus where two nearly copropagating molecular beams achieve an intersection angle of 4° at the interaction region. This near-copropagating beam geometry of the molecular beams permits very wide tuning of the collision energy, from far above room temperature down to 2 K where we test the theoretical treatment of the attractive part of the potentials and the difference potential for the first time. We have obtained differential cross sections for state-to-state collisions of NO (v = 10) with Ar and Ne in both spin-orbit manifolds using velocity map imaging. Overall good agreement of the experimental results was seen with quantum mechanical close-coupling calculations done on both coupled-cluster and multi-reference configuration interaction potential energy surfaces. Probing cold collisions of NO carrying ∼2 eV of vibrational excitation allows us to test state-of-the-art theory in this extreme nonequilibrium regime.

Differential Cross Sections for Cold, State-to-State Spin-Orbit Changing Collisions of NO(v = 10) with Neon

Inelastic scattering processes have proven a powerful means of investigating molecular interactions, and much current effort is focused on the cold and ultracold regime where quantum phenomena are clearly manifested. Studies of collisions of the open shell nitric oxide (NO) molecule have been central in this effort since the pioneering work of Houston and co-workers in the early 1990s. State-to-state scattering of vibrationally excited molecules in the cold regime introduces challenges that test the suitability of current theoretical methods for ab initio determination of intermolecular potentials, and concomitant electronically nonadiabatic processes raise the bar further. Here we report measurements of differential cross sections for state-to-state spin-orbit changing collisions of NO (v = 10, Ω″ = 1.5, and j″ = 1.5) with neon from 2.3 to 3.5 cm^-1 collision energy using our recently developed near-copropagating beam technique. The experimental results are compared with those obtained from quantum scattering calculations on a high-level set of coupled cluster potential energy surfaces and are shown to be in good agreement. The theoretical results suggest that distinct backscattering in the 2.3 cm^-1 case arises from overlapping resonances.

Harnessing the Power of Adiabatic Curve Crossing to Populate the Highly Vibrationally Excited H_{2} (v=7, j=0) Level

A large ensemble of ∼10^{9} H_{2} (v=7, j=0) molecules is prepared in the collision-free environment of a supersonic beam by transferring nearly the entire H_{2} (v=0, j=0) ground-state population, where v and j are the vibrational and rotational quantum numbers, respectively. This is accomplished by controlling the crossing of the optically dressed adiabatic states using a pair of phase coherent laser pulses. The preparation of highly vibrationally excited H_{2} molecules opens new opportunities to test fundamental physical principles using two loosely bound yet entangled H atoms.

Observation of an isomerizing double-well quantum system in the condensed phase

Molecular isomerization fundamentally involves quantum states bound within a potential energy function with multiple minima. For isolated gas-phase molecules, eigenstates well above the isomerization saddle points have been characterized. However, to observe the quantum nature of isomerization, systems in which transitions between the eigenstates occur-such as condensed-phase systems-must be studied. Efforts to resolve quantum states with spectroscopic tools are typically unsuccessful for such systems. An exception is CO adsorbed on NaCl(100), which is bound with the well-known OC-Na⁺ structure. We observe an unexpected upside-down isomer (CO-Na⁺) produced by infrared laser excitation and obtain well-resolved infrared fluorescence spectra from highly energetic vibrational states of both orientational isomers. This distinctive condensed-phase system is ideally suited to spectroscopic investigations of the quantum nature of isomerization.

Electron transfer mediates vibrational relaxation of CO in collisions with Ag(111)

We report vibrational relaxation probabilities for CO(v = 17) scattered from Ag(111) and compare our results to studies on other molecule–surface systems, which indicates a clear dependence of the relaxation probability on the work function of the surface and the electron binding energy of the molecule.

When is electronic friction reliable for dynamics at a molecule–metal interface?

Conditions under which electronic friction dynamics are applicable in the nonadiabatic limit are determined by examination of three model systems.

Ultrafast structural molecular dynamics investigated with 2D infrared spectroscopy methods

Ultrafast, multi-dimensional infrared (IR) spectroscopy has been advanced in recent years to a versatile analytical tool with a broad range of applications to elucidate molecular structure on ultrafast timescales, and it can be used for samples in a many different environments. Following a short and general introduction on the benefits of 2D IR spectroscopy, the first part of this chapter contains a brief discussion on basic descriptions and conceptual considerations of 2D IR spectroscopy. Outstanding classical applications of 2D IR are used afterwards to highlight the strengths and basic applicability of the method. This includes the identification of vibrational coupling in molecules, characterization of spectral diffusion dynamics, chemical exchange of chemical bond formation and breaking, as well as dynamics of intra- and intermolecular energy transfer for molecules in bulk solution and thin films. In the second part, several important, recently developed variants and new applications of 2D IR spectroscopy are introduced. These methods focus on (i) applications to molecules under two- and three-dimensional confinement, (ii) the combination of 2D IR with electrochemistry, (iii) ultrafast 2D IR in conjunction with diffraction-limited microscopy, (iv) several variants of non-equilibrium 2D IR spectroscopy such as transient 2D IR and 3D IR, and (v) extensions of the pump and probe spectral regions for multi-dimensional vibrational spectroscopy towards mixed vibrational-electronic spectroscopies. In light of these examples, the important open scientific and conceptual questions with regard to intra- and intermolecular dynamics are highlighted. Such questions can be tackled with the existing arsenal of experimental variants of 2D IR spectroscopy to promote the understanding of fundamentally new aspects in chemistry, biology and materials science. The final part of the chapter introduces several concepts of currently performed technical developments, which aim at exploiting 2D IR spectroscopy as an analytical tool. Such developments embrace the combination of 2D IR spectroscopy and plasmonic spectroscopy for ultrasensitive analytics, merging 2D IR spectroscopy with ultra-high-resolution microscopy (nanoscopy), future variants of transient 2D IR methods, or 2D IR in conjunction with microfluidics. It is expected that these techniques will allow for groundbreaking research in many new areas of natural sciences.

Vibrational Relaxation of Highly Vibrationally Excited CO Scattered from Au(111): Evidence for CO- Formation

Electronically nonadiabatic dynamics can be important in collisions of molecules at surfaces; for example, when vibrational degrees of freedom of molecules are coupled to electron-hole-pair (EHP) excitation of a metal. Such dynamics have been inferred from a host of observations involving multiquantum relaxation of NO molecules scattered from metal surfaces. Electron transfer forming transient NO^- is thought to be essential to the nonadiabatic coupling. The question remains: is this behavior usual? Here, we present final vibrational state distributions resulting from the scattering of CO(v_i = 17) from Au(111), which exhibits significantly less vibrational relaxation than NO(v_i = 16). We explain this observation in terms of the lower electron affinity of CO compared to NO, a result that is consistent with the formation of a transient CO^- ion being important to CO vibrational relaxation.

Vibrational ladder-climbing in surface-enhanced, ultrafast infrared spectroscopy

In a recent work (J. Phys. Chem. C 2016, 120, 3350-3359), we have introduced the concept of surface-enhanced, two-dimensional attenuated total reflectance (2D ATR IR) spectroscopy with modest enhancement factors (<50) using small plasmonic noble metal nanoparticles at solid-liquid interfaces. Here, we show that employment of almost continuous noble metal layers results in significantly stronger enhancement factors in 2D ATR IR signals (>450), which allows for multi-quantum IR excitation of adsorbed molecules, a process known as "vibrational ladder-climbing", even for weakly absorbing (ε < 200 M(-1) cm(-1)) nitrile IR labels. We show that it is possible to deposit up to four quanta of vibrational energy in the respective functional group. Based on these results, optical near-fields of plasmonic nanostructures may pave the way for future investigations involving ultrafast dynamics of highly excited vibrational states or surface-sensitive coherent control experiments of ground-state reactions at solid-liquid interfaces.

Communication: Observation of local-bender eigenstates in acetylene

We report the observation of eigenstates that embody large-amplitude, local-bending vibrational motion in acetylene by stimulated emission pumping spectroscopy via vibrational levels of the S1 state involving excitation in the non-totally symmetric bending modes. The N(b) = 14 level, lying at 8971.69 cm(-1) (J = 0), is assigned on the basis of degeneracy due to dynamical symmetry breaking in the local-mode limit. The level pattern for the N(b) = 16 level, lying at 10 218.9 cm(-1), is consistent with expectations for increased separation of ℓ = 0 and 2 vibrational angular momentum components. Increasingly poor agreement between our observations and the predicted positions of these levels highlights the failure of currently available normal mode effective Hamiltonian models to extrapolate to regions of the potential energy surface involving large-amplitude displacement along the acetylene ⇌ vinylidene isomerization coordinate.

A new Stark decelerator based surface scattering instrument for studying energy transfer at the gas-surface interface

We report on the design and characterization of a new apparatus for performing quantum-state resolved surface scattering experiments. The apparatus combines optical state-specific molecule preparation with a compact hexapole and a Stark decelerator to prepare carrier gas-free pulses of quantum-state pure CO molecules with velocities controllable between 33 and 1000 m/s with extremely narrow velocity distributions. The ultrahigh vacuum surface scattering chamber includes homebuilt ion and electron detectors, a closed-cycle helium cooled single crystal sample mount capable of tuning surface temperature between 19 and 1337 K, a Kelvin probe for non-destructive work function measurements, a precision leak valve manifold for targeted adsorbate deposition, an inexpensive quadrupole mass spectrometer modified to perform high resolution temperature programmed desorption experiments and facilities to clean and characterize the surface.

Electron transfer rate modulation in a compact Re(i) donor–acceptor complex

Excitation of the vibrational modes at the bipyridine ligand results in modulation of the electron transfer rate between the electron donating and accepting ligands in a Re(i) complex.

Dynamical traps lead to the slowing down of intramolecular vibrational energy flow

The phenomenon of intramolecular vibrational energy redistribution (IVR) is at the heart of chemical reaction dynamics. Statistical rate theories, assuming instantaneous IVR, predict exponential decay of the population with the properties of the transition state essentially determining the mechanism. However, there is growing evidence that IVR competes with the reaction timescales, resulting in deviations from the exponential rate law. Dynamics cannot be ignored in such cases for understanding the reaction mechanisms. Significant insights in this context have come from the state space model of IVR, which predicts power law behavior for the rates with the power law exponent, an effective state space dimensionality, being a measure of the nature and extent of the IVR dynamics. However, whether the effective IVR dimensionality can vary with time and whether the mechanism for the variation is of purely quantum or classical origins are issues that remain unresolved. Such multiple power law scalings can lead to surprising mode specificity in the system, even above the threshold for facile IVR. In this work, choosing the well-studied thiophosgene molecule as an example, we establish the anisotropic and anomalous nature of the quantum IVR dynamics and show that multiple power law scalings do manifest in the system. More importantly, we show that the mechanism of the observed multiple power law scaling has classical origins due to a combination of trapping near resonance junctions in the network of classical nonlinear resonances at short to intermediate times and the influence of weak higher-order resonances at relatively longer times.

Standoff trace chemical sensing via manipulation of excited electronic state lifetimes

We present a technique for standoff trace chemical sensing that is based on the dependence of excited electronic state lifetimes on the amount of internal vibrational energy. The feasibility of the technique is demonstrated using N,N-dimethylisopropylamine (DMIPA). Time-resolved measurements show that the lifetime of the S1 state in DMIPA exponentially decreases with the amount of vibrational energy. This property is employed to acquire molecular spectral signatures. Two laser pulses are used: one ionizes the molecule through the S1 state; the other alters the S1 state lifetime by depositing energy into vibrations. Reduction of the S1 state lifetime decreases ionization efficiency that is observed by probing the laser-induced plasma with microwave radiation.

Energy-dependence of vibrational relaxation between highly vibrationally excited KH (X1Σ+, ν"=14-23) and H2, and N2

Vibrational state total relaxation rate coefficients, k(ν") (M), for KH (ν"=14-23) by M=H(2) and N(2) have been investigated in an overtone pump-probe configuration. At ν"=14, 15, 16 and 17, the rate coefficients k(ν)(″) (M) increase linearly with vibrational quantum number. The region (ν"=18, 19, 20 and 21) where the dependence is much stronger than linear has significant contribution from multiquantum (Δν≥2) relaxation. For ν"=18, 19, 20 and 21, 0.25, 0.31, 0.38 and 0.31 of the initially prepared population undergo two-quantum (Δν=2) vibrational relaxation in KH (ν")+H(2) collisions. In KH (ν")+N(2), the time profile of ν"=14(15) after preparation of ν"=19(20) was measured. A clear bimodal distribution is observed. The time scale of the first peak is much shorter than the known collisional lifetimes of the intervening vibrational levels and thus a sequential single-quantum relaxation mechanism can be explicitly ruled out. Relaxation of KD with D(2) has been also investigated. The relaxation rate coefficients exhibit distinct maxima for both isotopes (KH and KD). We discuss possible explanation of the experimental results including mass effect, V-R energy transfer and V-V energy transfer.

Electron kinetic energies from vibrationally promoted surface exoemission: evidence for a vibrational autodetachment mechanism

We report kinetic energy distributions of exoelectrons produced by collisions of highly vibrationally excited NO molecules with a low work function Cs dosed Au(111) surface. These measurements show that energy dissipation pathways involving nonadiabatic conversion of vibrational energy to electronic energy can result in electronic excitation of more than 3 eV, consistent with the available vibrational energy. We measured the dependence of the electron energy distributions on the translational and vibrational energy of the incident NO and find a clear positive correlation between final electron kinetic energy and initial vibrational excitation and a weak but observable inverse dependence of electron kinetic energy on initial translational energy. These observations are consistent with a vibrational autodetachment mechanism, where an electron is transferred to NO near its outer vibrational turning point and ejected near its inner vibrational turning point. Within the context of this model, we estimate the NO-to-surface distance for electron transfer.

Dynamics of molecules in extreme rotational states

We have constructed an optical centrifuge with a pulse energy that is more than 2 orders of magnitude larger than previously reported instruments. This high pulse energy enables us to create large enough number densities of molecules in extreme rotational states to perform high-resolution state-resolved transient IR absorption measurements. Here we report the first studies of energy transfer dynamics involving molecules in extreme rotational states. In these studies, the optical centrifuge drives CO2 molecules into states with J ∼ 220 and we use transient IR probing to monitor the subsequent rotational, translational, and vibrational energy flow dynamics. The results reported here provide the first molecular insights into the relaxation of molecules with rotational energy that is comparable to that of a chemical bond.

Molecules: what kind of a bag of atoms?

As discussed by Liang and Dill, Enright and Leitner, and others, proteins are not 3D objects. We study an expanded macromolecular data set ranging from proteins to RNA, lipids, and viruses, and remove surface effects and size bias. Molecules and molecular assemblies with more than 1000 backbone atoms have a volume fractal dimension of D(v) = 2.70 +/- 0.05 by the embedded sphere method and D(v) = 2.71 +/- 0.04 by the ensemble method using radius of gyration as the size measure. The much larger D(v) = 2.89 +/- 0.05 obtained with the average surface radius as the length measure shows that surface corrugation is as extensive as cavity formation. Using a simple "Swiss cheese" model for molecules, we show that the distribution of voids in the interior of molecules cannot be a Boltzmann distribution of void energy as a function of void size. Instead, frustration from imperfect packing builds up with molecular size, allowing larger voids to form in larger molecules. We find that large molecules lie halfway between the extremes of packing for homogeneous objects (D = 3) and Apollonian packing, which accounts for packing of a hierarchy of random-sized objects (D approximately 2.47).

Efficient vibrational and translational excitations of a solid metal surface: State-to-state time-of-flight measurements of HCl(v=2,J=1) scattering from Au(111)

We report high resolution state-to-state time-of-flight (TOF) measurements for scattering of HCl(v=2, J=1) from a Au(111) single crystal surface for both vibrationally elastic (v=2-->2) as well as inelastic (v=2-->1) channels at seven incidence energies between 0.28 and 1.27 eV. The dependences of the TOF results on final HCl rotational state and surface temperature are also reported. The translational energy transferred to the surface depends linearly on incidence energy and is close to the single surface-atom impulse (Baule) limit over the entire range of incidence energies studied. The probability of vibrational relaxation is also large. For molecules that relax from v=2 to v=1, the fraction of vibrational energy that is transferred to the surface is approximately 74%. We discuss these observations in terms of an impulse approximation as well as the possible role of translational and vibrational excitations of electron-hole pairs in the solid.

Kinetic study of vibrational energy transfer from a wide range of vibrational levels of O2(X(3)Sigma(g)-, v = 6-12) to CF4

A wide range of vibrational levels of O2(X(3)Sigma(g)(-), v = 6-13) generated in the ultraviolet photolysis of O3 was selectively detected by the laser-induced fluorescence (LIF) technique. The time-resolved LIF-excited B(3)Sigma(u)(-)-X(3)Sigma(g)(-) system in the presence of CF4 has been recorded and analyzed by the integrated profiles method (IPM). The IPM permitted us to determine the rate coefficients k(v)(CF4) for vibrational relaxation of O2(X(3)Sigma(g)(-), v = 6-12) by collisions with CF4. Energy transfer from O2 (v = 6-12) to CF4 is surprisingly efficient compared to that of other polyatomic relaxation partners studied so far. The k(v)(CF4) increases with vibrational quantum number v from [1.5 +/- 0.2(2sigma)] x 10(-12) for v = 6 to [7.3 +/- 1.5(2sigma)] x 10(-11) for v = 12, indicating that the infrared-active nu3 vibrational mode of CF4 mainly governs the energy transfer with O2(X(3)Sigma(g)(-), v = 6-12). The correlation between the rate coefficients and fundamental infrared intensities has been discussed based on a comparison of the efficiency of energy transfer by several collision partners.

Coherent manipulations of atoms using laser light

The internal structure of a particle - an atom or other quantum system in which the excitation energies are discrete - undergoes change when exposed to pulses of near-resonant laser light. This tutorial review presents basic concepts of quantum states, of laser radiation and of the Hilbert-space statevector that provides the theoretical portrait of probability amplitudes - the tools for quantifying quantum properties not only of individual atoms and molecules but also of artificial atoms and other quantum systems. It discusses the equations of motion that describe the laser-induced changes (coherent excitation), and gives examples of laser-pulse effects, with particular emphasis on two-state and three-state adiabatic time evolution within the rotating-wave approximation. It provides pictorial descriptions of excitation based on the Bloch equations that allow visualization of two-state excitation as motion of a three-dimensional vector (the Bloch vector). Other visualization techniques allow portrayal of more elaborate systems, particularly the Hilbert-space motion of adiabatic states subject to various pulse sequences. Various more general multilevel systems receive treatment that includes degeneracies, chains and loop linkages. The concluding sections discuss techniques for creating arbitrary pre-assigned quantum states, for manipulating them into alternative coherent superpositions and for analyzing an unknown superposition. Appendices review some basic mathematical concepts and provide further details of the theoretical formalism, including photons, pulse propagation, statistical averages, analytic solutions to the equations of motion, exact solutions of periodic Hamiltonians, and population-trapping "dark" states.