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Water can be vitrified if it is cooled at high rates, which makes it possible to outrun crystallization in so-called no man's land, a range of deeply supercooled temperatures where water crystallizes rapidly. Here, we study the reverse process in pure water samples by flash melting amorphous ice with microsecond laser pulses. Time-resolved electron diffraction reveals that the sample transiently crystallizes despite a heating rate of more than 5 × 106 K/s, even though under the same conditions, vitrification can be achieved with a similar cooling rate of 107 K/s. Moreover, we observe different crystallization kinetics for amorphous solid water and hyperquenched glassy water. These experiments open up new avenues for elucidating the crystallization mechanism of water and studying its dynamics in no man's land. They also add important insights into the laser melting and revitrification processes that are integral to the emerging field of microsecond time-resolved cryo-electron microscopy. [ABSTRACT FROM AUTHOR]

We present a joint experimental and theoretical study on the desolvation of Ba+ cations in 4He nanodroplets excited via the 6p ← 6s transition. The experiments reveal an efficient desolvation process yielding mainly bare Ba+ cations and Ba+Hen exciplexes with n = 1 and 2. The speed distributions of the ions are well described by Maxwell-Boltzmann distributions with temperatures ranging from 60 to 178 K depending on the excitation frequency and Ba+ Hen exciplex size. These results have been analyzed by calculations based on a time-dependent density functional description for the helium droplet combined with classical dynamics for the Ba+. In agreement with experiment, the calculations reveal the dynamical formation of exciplexes following excitation of the Ba+ cation. In contrast to experimental observation, the calculations do not reveal desolvation of excited Ba+ cations or exciplexes, even when relaxation pathways to lower lying states are included. [ABSTRACT FROM AUTHOR]

Whereas most of the phenomena associated with superfluidity have been observed in finite-size helium systems, the nucleation of quantized vortices has proven elusive. Here we show using timedependent density functional simulations that the solvation of a Ba+ ion created by photoionization of neutral Ba at the surface of a 4He nanodroplet leads to the nucleation of a quantized ring vortex. The vortex is nucleated on a 10 ps timescale at the equator of a solid-like solvation structure that forms around the Ba+ ion. The process is expected to be quite general and very efficient under standard experimental conditions. [ABSTRACT FROM AUTHOR]

The solvation of Ba+ ions created by the photoionization of barium atoms located on the surface of helium nanodroplets has been investigated. The excitation spectra corresponding to the 6p 2P1/2 ← 6s 2S1/2 and 6p 2P3/2 ← 6s 2S1/2 transitions of Ba+ are found to be identical to those recorded in bulk He II [H. J. Reyher, H. Bauer, C. Huber, R. Mayer, A. Schafer, and A. Winnacker, Phys. Lett. A 115, 238 (1986)], indicating that the ions formed at the surface of the helium droplets become fully solvated by the helium. Time-of-flight mass spectra suggest that following the excitation of the solvated Ba+ ions, these are being ejected from the helium droplets either as bare Ba+ ions or as small Ba+Hen (n < 20) complexes. [ABSTRACT FROM AUTHOR]

Excitation spectra up to the ionization threshold are reported for barium atoms located on the surface of helium nanodroplets. For states with low principal quantum number, the resonances are substantially broadened and shifted towards higher energy with respect to the gas phase. This has been attributed to the repulsive interaction of the excited atom with the helium at the Franck-Condon region. In contrast, for states with high principal quantum number the resonances are narrower and shifted towards lower energies. Photoelectron and ZEKE spectroscopy reveal that the redshift results from a lowering of the ionization threshold due to polarization of the helium by the barium ionic core. As a result of the repulsive interaction with the helium, excited barium atoms desorb from the surface of the droplets. Only when excited to the 6s6p 1P1 state, which reveals an attractive interaction with the helium, the atoms remain attached to the droplets. [ABSTRACT FROM AUTHOR]

High resolution IR spectra of aniline, styrene, and 1,1-diphenylethylene cations embedded in superfluid helium nanodroplets have been recorded in the 300-1700 cm-1 range using a free-electron laser as radiation source. Comparison of the spectra with available gas phase data reveals that the helium environment induces no significant matrix shift nor leads to an observable line broadening of the resonances. In addition, the IR spectra have provided new and improved vibrational transition frequencies for the cations investigated, as well as for neutral aniline and styrene. Indications have been found that the ions desolvate from the droplets after excitation by a non-evaporative process in which they are ejected from the helium droplets. The kinetic energy of the ejected ions is found to be ion specific and to depend only weakly on the excitation energy. [ABSTRACT FROM AUTHOR]

The photodissociation of (fluorinated) alkyl iodides in helium nanodroplets at a wavelength of 266 nm has been investigated by means of ion imaging techniques. It is found that a significant fraction of the created fragments escapes from the helium droplets. The speed and kinetic energy distributions of these fragments are found to be notably modified with respect to the corresponding gas phase distributions. The fragments, furthermore, show a speed dependent angular distribution. The loss of kinetic energy as well as the reduction of the anisotropy parameter show a strong mass dependence. These observations point to a nonthermal escape process in which the kinetic energy and momentum transfer from the fragments to the solvent is governed by binary collisions with the individual helium atoms making up the droplet. Monte Carlo simulations based on hard-sphere binary collisions substantiate this interpretation of the data. [ABSTRACT FROM AUTHOR]

The solvation dynamics of nonthermal species in liquid helium has been investigated by photolyzing alkyl iodide molecules, CH3I, C2H5I, and CF3I, embedded in helium nanodroplets. Iodine and CH3 fragments are found to leave the droplets solvated by a finite number of helium atoms, this in contrast to C2H5 and CF3 fragments. The speed distributions of the IHeN and CH3HeN complexes show a prominent correlation with the degree of solvation N. It is argued that this correlation is caused by a dynamical adjustment of the solvation structure size to the relative speed of the traveling fragments as they pass through the helium bath. The absence of C2H5HeN and CF3HeN complexes is attributed to the large internal energy of these alkyl fragments which leads to a rapid destruction of any possibly formed complexes. [ABSTRACT FROM AUTHOR]

The recombination of fragments resulting from the photodissociation of (fluorinated) alkyl iodides in helium nanodroplets at a wavelength of 266 nm has been investigated by means of ion imaging techniques. It is found that in the case of CH3I an appreciable fraction of the fragments recombine in the aftermath of the photolysis. The proposed mechanism involves a complete translational relaxation of both photofragments inside the nanodroplets followed by geminate recombination of the fragments. In contrast with CH3I, no recombination is observed for CF3I. This is attributed to the larger masses and the different initial kinetic energies of the fragments produced by the photolysis of CF3I, which strongly diminishes the fragment thermalization efficiency. [ABSTRACT FROM AUTHOR]

Fully rotationally resolved spectra of the two lowest-frequency bands (the origin (0[sub 0][sup 0]) and the inversion (14[sub 0][sup 1]) band) of the S[sub 1](nπ[sup *])←S[sub 0] transition of jet-cooled (T[sub rot]approx. 6 K) acetaldehyde, CH[sub 3]CHO, have been recorded with a resolution of approx. 0.01 cm-1 using a pulsed dye amplified continuous wave (cw) laser. In modeling the spectra a nonperturbative solution of the rotational-torsional coupling problem was used, but the torsion-inversion and the rotation-inversion couplings were neglected. All the lines of the origin band were reproduced with a model using the same rotational-torsional Hamiltonian for the ground- and excited electronic state. The inversion band could not be described with this model, since the ordering of the torsional levels in the excited state is reversed. The measured spectrum was reproduced by using a rigid asymmetric rotor Hamiltonian for the two torsional levels in the excited state. Some rotational levels of 0[sup 0] and 14[sup 1] states were found to be shifted from their predicted energy values. These shifts were explained by an accidental resonance between the excited singlet level and some higher rovibronic triplet level. The relative intensities of the ab-type and c-type torsional subbands and the Herzberg-Teller-induced transition dipole moment direction, characterizing the ab-type subband, were determined by an intensity fit. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Metastable time-of-flight (TOF) spectroscopy was used to measure the translational energy distribution of specific rotational states of CO formed from ketene photodissociation (CH2CO→CH2+CO) at 351 nm. This distribution could be directly related to the internal energy distribution of the other fragment (X 3B1 CH2) formed in the reaction, thereby giving a correlated distribution of the internal states of the fragments. This technique overcomes the spectral complexity associated with detection the X 3B1 state CH2. Previous measurements of the CO rotational distribution were simulated theoretically using the impulsive model and zero-point vibrational energy considerations. These models predicted that the rotational distributions of CO and CH2 should be uncorrelated, that ∼10% of the CH2 should be vibrationally excited with one quantum in the bending mode, and that the rotational energy distribution of CH2 should peak near zero. Measurements presented in this paper show a slight anticorrelation of CO and CH2 rotations, no vibrational excitation of CH2 and Gaussian-like rotational energy distributions of CH2 that peak at ∼1 kcal/mole and have a full width at half-maximum of ∼0.8 kcal/mol. Qualitative explanations for this behavior are presented. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The correlated product state distribution for ketene photodissociation (CH2CO→CH2+CO) at 308 nm has been measured by using quantum-state-specific metastable time-of-flight (TOF) spectroscopy. This distribution is a matrix whose elements are the probability that if CO is produced in the dissociation with quantum-state |nCO>, CH2 will be produced with quantum-state |nCH2>. It was found that ketene photodissociation yields CH2 in three resolved states; the 1A1(000), and 1A1(010) states of CH2 are the major channels, while the 3B1 state is a minor channel. In addition to this scalar distribution, the vector correlations between the recoil velocity and the angular momentum of the CO fragment (v·j correlation), expressed by the β00(22) bipolar moment, have also been obtained as a function of the kinetic energy release of the photoreaction. The correlated product state distribution was found not to follow the predictions of phase space theory, suggesting that dynamic hindrances exist in the photoreaction that have not been previously observed. A phase space theory calculation with restricted impact parameter values was also performed and compared to experiment. The impact parameter restricted phase space theory more accurately reproduced all of the correlated product state information obtained in this work as well as previous uncorrelated product state distributions for CH2 and CO. Both the ranges and the values of the allowed impact parameters obtained from these restricted calculations increase as the rotational energy of CO increases. Also, the values of the allowed impact parameters for 1A1(010) CH2 are larger than for 1A1(000) CH2. This strongly suggests that C–C–O bending modes are hindered at the transition state and therefore play an important role in the photodissociation. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A new scheme for performing high-resolution kinetic energy measurements on single quantum states of NO is described. Laser excitation of the a 4Π(ν=13–15) and b 4Σ-(ν=3)←X 2Π(ν=0) spin-forbidden bands has been used to produce NO a 4Π, either by direct excitation or after excitation to NO b 4Σ- followed by b→a emission. The sensitivity is compared to previous experiments on CO, employing the forbidden Cameron system. In the course of these experiments, three previously unknown vibronic levels of the a 4Π state were observed for which high precision molecular constants have been obtained. The derived molecular constants have been used to construct a chemically accurate RKR potential for the a 4Π state to within 0.2 eV of the dissociation limit. In addition, the electric dipole moments for the a 4Π(ν=13–14) levels have been determined. By comparing the experimental results with two ab initio calculations, the polarity of the dipole moment of the a 4Π state at equilibrium separation could be determined. A set of supplementary molecular beam laser-induced fluorescence studies on the b 4Σ-(ν=3)←X 2Π(ν=0) band were also carried out. These allowed the fluorescence lifetime of b 4Σ-(ν=3) state to be obtained under collision-free conditions and the electronic origin of the quartet manifold to be determined within 0.03 cm-1. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A new method is demonstrated which allows derivation of elements of the correlated-product-state distribution matrix in photodissociation. The method relies on quantum-state-specific, time-of-flight measurements, which were successfully carried out on CO produced in ketene photolysis at 308 nm. Such time-of-flight data exhibits well-defined peaks dictated by the CH2 internal state distribution occurring in coincidence with the chosen CO quantum state. For two rovibrational states of CO, peaks corresponding to the 1A1(0,0,0), 1A1(0,1,0), and 3B1 vibronic states of the CH2 fragment were clearly resolved. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Laser-induced fluorescence spectra of the 330 K10 and 340 K10 vibronic bands of the A 1Au←X 1Σ+g transition in acetylene have been recorded with a resolution of 18 MHz. Each rotational transition consists of a group of lines due to coupling of the electronically excited singlet state with isoenergetic triplet states. Using the standard deconvolution procedure the singlet–triplet coupling elements and the density of coupled triplet states are derived for rotational levels up to J=4 in both bands. From the density of coupled triplet states it is concluded that the A 1Au state is perturbed by the T1 3B2 state. Magnetic field measurements have shown that the predissociation of acetylene in the 4ν3’ vibrational level of the A state is caused by a coupling via the T1 3B2 state with predissociating vibrational levels of the electronic ground state. [ABSTRACT FROM AUTHOR]

The Rydberg states L 1Π(v=0), L’ 1Π(v=1), K 1Σ+(v=0), W 1Π(v=0), and W’ 1Π(v=2) have been studied in a 2+1 double-resonance detection scheme with a resolution of 0.005 cm-1. Accurate molecular constants have been derived for these states. From the observed linewidths of individual rotational transitions, predissociation rates for the Rydberg states have been deduced. For the first time a clear J and e/f dependence for the predissociation of CO has been observed. One of the states causing the observed predissociation could be identified as the D’ 1Σ+ state. [ABSTRACT FROM AUTHOR]

Transitions from the X 1Σ+(v=0) ground state of the carbon monoxide molecule to the electronically excited A 1Π(v=0), B 1Σ+(v=0), and C 1Σ+(v=0) states have been studied by 2-photon laser induced fluorescence spectroscopy. Accurate molecular constants for the B and C state have been determined. The electric dipole moments for all three electronically excited states have been deduced from the observed Stark effects. The dipole moments for the A, B, and C states are found to be 0.335±0.013, 1.95±0.03, and 4.50±0.07 D, respectively. From the observed radiative lifetimes the transition probabilities of the B–A, B–X and C–B, C–A, and C–X transitions have been determined. [ABSTRACT FROM AUTHOR]

We report the measurement of seven rovibronic bands belonging to the C3Π(v’=0–6)-X3Π(v‘=0) transition in SiC. The molecules are produced by laser vaporization in combination with supersonic cooling and the transitions are detected using pulsed laser induced fluorescence spectroscopy. The vibrational and rotational structure of the C3Π state are determined. Radiative lifetimes vary from 2886 ns for v’=0 to 499 ns for v’=6. Dispersed fluorescence spectra from the excited vibrational levels to the ground state have been measured and the vibrational constants of the electronic ground state were obtained. Both the v’ dependence of the radiative lifetime and observed anomalous intensity distributions in the fluorescence spectra are explained by an electronic transition moment function that varies with the internuclear distance. The results are compared with ab initio calculations. [ABSTRACT FROM AUTHOR]

Investigates the compensation of rotational motion on the energy defect in near-resonant vibration-vibration energy transfer. Observation of vibrationally elastic energy transfer; Dependence of process efficiency on energy defect; Influence of vibration on rotational energy transfer.

Presents an experimental method for measurement of collision-free infrared radiative lifetimes for single quantum states of a vibrationally excited sample. Provision of a more direct route to the infrared Einstein A coefficients; Derivation of the electric dipole moment function.

The possibility for helium-induced electronic transitions in a photo-excited atom is investigated using Ba+ excited to the 6p 2P state as a prototypical example. A diabatization scheme has been designed to obtain the necessary potential energy surfaces and couplings for complexes of Ba+ with an arbitrary number of helium atoms. It involves computing new He–Ba+ electronic wave functions and expanding them in determinants of the non-interacting complex. The 6p 2P ← 6s 2S photodissociation spectrum of He⋯Ba+ calculated with this model shows very weak coupling for a single He atom. However, several electronic relaxation mechanisms are identified, which could potentially explain the expulsion of barium ions from helium nanodroplets observed experimentally upon Ba+ photoexcitation. For instance, an avoided crossing in the ring-shaped He7Ba+ structure is shown to provide an efficient pathway for fine structure relaxation. Symmetry breaking by either helium density fluctuations or vibrations can also induce efficient relaxation in these systems, e.g., bending vibrations in the linear He2Ba+ excimer. The identified relaxation mechanisms can provide insight into helium-induced non-adiabatic transitions observed in other systems. [ABSTRACT FROM AUTHOR]

The possibility for helium-induced electronic transitions in a photo-excited atom is investigated using Ba + excited to the 6p 2 P state as a prototypical example. A diabatization scheme has been designed to obtain the necessary potential energy surfaces and couplings for complexes of Ba + with an arbitrary number of helium atoms. It involves computing new He-Ba + electronic wave functions and expanding them in determinants of the non-interacting complex. The 6p 2 P ← 6s 2 S photodissociation spectrum of He⋯Ba + calculated with this model shows very weak coupling for a single He atom. However, several electronic relaxation mechanisms are identified, which could potentially explain the expulsion of barium ions from helium nanodroplets observed experimentally upon Ba + photoexcitation. For instance, an avoided crossing in the ring-shaped He 7 Ba + structure is shown to provide an efficient pathway for fine structure relaxation. Symmetry breaking by either helium density fluctuations or vibrations can also induce efficient relaxation in these systems, e.g., bending vibrations in the linear He 2 Ba + excimer. The identified relaxation mechanisms can provide insight into helium-induced non-adiabatic transitions observed in other systems.

We present a joint experimental and theoretical study on the desolvation of Ba(+) cations in (4)He nanodroplets excited via the 6p ← 6s transition. The experiments reveal an efficient desolvation process yielding mainly bare Ba(+) cations and Ba(+)Hen exciplexes with n = 1 and 2. The speed distributions of the ions are well described by Maxwell-Boltzmann distributions with temperatures ranging from 60 to 178 K depending on the excitation frequency and Ba(+) Hen exciplex size. These results have been analyzed by calculations based on a time-dependent density functional description for the helium droplet combined with classical dynamics for the Ba(+). In agreement with experiment, the calculations reveal the dynamical formation of exciplexes following excitation of the Ba(+) cation. In contrast to experimental observation, the calculations do not reveal desolvation of excited Ba(+) cations or exciplexes, even when relaxation pathways to lower lying states are included.

Whereas most of the phenomena associated with superfluidity have been observed in finite-size helium systems, the nucleation of quantized vortices has proven elusive. Here we show using time-dependent density functional simulations that the solvation of a Ba(+) ion created by photoionization of neutral Ba at the surface of a (4)He nanodroplet leads to the nucleation of a quantized ring vortex. The vortex is nucleated on a 10 ps timescale at the equator of a solid-like solvation structure that forms around the Ba(+) ion. The process is expected to be quite general and very efficient under standard experimental conditions.