Search

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]

A microsecond time‐resolved version of cryo‐electron microscopy (cryo‐EM) has recently been introduced to enable observation of the fast conformational motions of proteins. The technique involves locally melting a cryo sample with a laser beam to allow the proteins to undergo dynamics in the liquid phase. When the laser is switched off, the sample cools within just a few microseconds and revitrifies, trapping particles in their transient configurations, in which they can subsequently be imaged. Two alternative implementations of the technique have previously been described, using either an optical microscope or performing revitrification experiments in situ. Here, it is shown that it is possible to obtain near‐atomic resolution reconstructions from in situ revitrified cryo samples. Moreover, the resulting map is indistinguishable from that obtained from a conventional sample within the spatial resolution. Interestingly, it is observed that revitrification leads to a more homogeneous angular distribution of the particles, suggesting that revitrification may potentially be used to overcome issues of preferred particle orientation. [ABSTRACT FROM AUTHOR]

A novel approach to time‐resolved cryo‐electron microscopy (cryo‐EM) has recently been introduced that involves melting a cryo sample with a laser beam to allow protein dynamics to briefly occur in the liquid, before trapping the particles in their transient configurations by rapidly revitrifying the sample. With a time resolution of just a few microseconds, this approach is notably fast enough to study the domain motions that are typically associated with the activity of proteins but which have previously remained inaccessible. Here, crucial details are added to the characterization of the method. It is shown that single‐particle reconstructions of apoferritin and Cowpea chlorotic mottle virus from revitrified samples are indistinguishable from those from conventional samples, demonstrating that melting and revitrification leaves the particles intact and that they do not undergo structural changes within the spatial resolution afforded by the instrument. How rapid revitrification affects the properties of the ice is also characterized, showing that revitrified samples exhibit comparable amounts of beam‐induced motion. The results pave the way for microsecond time‐resolved studies of the conformational dynamics of proteins and open up new avenues to study the vitrification process and to address beam‐induced specimen movement. [ABSTRACT FROM AUTHOR]

The excited state dynamics of silver atoms embedded in helium nanodroplets were investigated by various spectroscopic techniques.

In ultrafast transmission electron microscopy, time zero can be accurately determined by making use of the photon-induced near-field electron microscopy (PINEM) effect, which causes electrons interacting with the near fields of a nanoparticle to coherently gain or lose energy in multiples of the photon energy when the laser pump and electron probe pulse overlap in time. If the instrument is not equipped with an energy filter, which is required to observe the PINEM effect, the response of a sample is frequently monitored instead. However, the gradual or delayed onset of this response can render an accurate measurement as challenging. Here, we demonstrate a simple and accurate method for determining time zero without an energy filter that is based on the observation that the outline of a nanoparticle blurs when the electron and laser pulse overlap in time. We show that this phenomenon arises from the PINEM effect, which causes some electrons to gain a large energy spread, thus blurring the image due to the chromatic aberration of the imaging system. This effect can also be used to characterize the instrument response and determine the laser polarization in situ. Furthermore, it may find applications for mapping out the near fields of a nanoparticle without the help of an energy filter. [ABSTRACT FROM AUTHOR]

An atomic infrared (IR) streak camera that operates in the mid- and far-infrared well beyond the long wavelength cutoff of conventional streak cameras is presented. The temporal and spectral characteristics of the streak camera are determined using the FELIX free-electron laser as the IR light source. The high sensitivity of the streak camera is demonstrated by single-shot characterization of the IR pulses of FELIX.

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]

The dynamics of proteins that are associated with their function typically occur on the microsecond timescale, orders of magnitude faster than the time resolution of cryo-electron microscopy. We have recently introduced a novel approach to time-resolved cryo-electron microscopy that affords microsecond time resolution. It involves melting a cryo sample with a heating laser, so as to allow dynamics of the proteins to briefly occur in the liquid phase. When the laser is turned off, the sample rapidly revitrifies, trapping the particles in their transient configurations. Precise control of the temperature evolution of the sample is crucial for such an approach to succeed. Here, we provide a detailed characterization of the heat transfer occurring under laser irradiation as well as the associated phase behavior of the cryo sample. While areas close to the laser focus undergo melting and revitrification, surrounding regions crystallize. In situ observations of these phase changes therefore provide a convenient approach for assessing the temperature reached in each melting and revitrification experiment and for adjusting the heating laser power on the fly. [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 rapid growth of the field of time-resolved and ultrafast electron microscopy has been accompanied by the active development of new instrumentation. Recently, time-resolved microscopes equipped with a field emission gun have been introduced, demonstrating great potential for experiments that benefit from the high brightness and coherence of the electron source. Here, we describe a straightforward design of a time-resolved transmission electron microscope with a Schottky field emission gun and characterize its performance. At the same time, our design gives us the flexibility to alternatively operate the instrument as if it was equipped with a flat metal photocathode. We can, thus, effectively choose to sacrifice brightness in order to obtain pulses with vastly larger numbers of electrons than from the emitter if for a given application the number of electrons is a crucial figure of merit. We believe that our straightforward and flexible design will be of great practical relevance to researchers wishing to enter the field. [ABSTRACT FROM AUTHOR]

Thanks to their high brightness, field emitters are the electron sources of choice in most high-end electron microscopes. Under typical operating conditions, the available emission current from these emitters is largely limited by practical considerations, and extracting significantly larger currents is usually not possible without reducing the lifetime of the emitter or even damaging it. Such limitations may, however, not apply if the emitter is only briefly subjected to extreme operating conditions so that damage can be outrun. Here, we demonstrate that it is possible to temporarily operate a Schottky emitter far outside its stable operating regime and significantly increase its emission current. We do so by locally heating the tip of the emitter with a microsecond laser pulse, which boosts the emission current by a factor of 3.7 to nearly 450 μA. We believe that the generation of intense microsecond electron pulses from a field emitter will particularly benefit the atomic-resolution imaging of fast processes that occur on the microsecond timescale. [ABSTRACT FROM AUTHOR]

High intensity XUV radiation from a free-electron laser (FEL) was used to create a nanoplasma inside ammonia clusters with the intent of studying the resulting electron–ion interactions and their interplay with plasma evolution. In a plasma-like state, electrons with kinetic energy lower than the local collective Coulomb potential of the positive ionic core are trapped in the cluster and take part in secondary processes (e.g. electron-impact excitation/ionization and electron–ion recombination) which lead to subsequent excited and neutral molecular fragmentation. Using a time-delayed UV laser, the dynamics of the excited atomic and molecular states are probed from −0.1 ps to 18 ps. We identify three different phases of molecular fragmentation that are clearly distinguished by the effect of the probe laser on the ionic and electronic yield. We propose a simple model to rationalize our data and further identify two separate channels leading to the formation of excited hydrogen. [ABSTRACT FROM AUTHOR]

The manipulation of liquids at nanoscale dimensions is a central goal of the emergent nanofluidics field. Such endeavors extend to nanodroplets, which are ubiquitous objects involved in many technological applications. Here, we employ time-resolved electron microscopy to elucidate the formation of so-called jumping nanodroplets on a graphene surface. We flash-melt a thin gold nanostructure with a laser pulse and directly observe how the resulting nanodroplet contracts into a sphere and jumps off its substrate, a process that occurs in just a few nanoseconds. Our study provides the first experimental characterization of these morphological dynamics through real-time observation and reveals new aspects of the phenomenon. We observe that friction alters the trajectories of individual droplets. Surprisingly, this leads some droplets to adopt dumbbell-shaped geometries after they jump, suggesting that they spin with considerable angular momentum. Our experiments open up new avenues for studying and controlling the fast morphological dynamics of nanodroplets through their interaction with structured surfaces. [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]

A novel spectroscopic method is described that has used superfluid helium nanodroplets to record IR spectra of cold molecular ions. The spectra are recorded with a high sensitivity, have displayed only a small matrix shift and the widths of the individual transitions have depended on the excited vibrational level.

Several high-resolution excitation and absorption spectroscopy studies are conducted to study the properties of the gas-phase p-coumaric acid. The analyses have been shown to be extremely useful for analyzing the light-induced signal transduction taking place in such complexes.

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 Low Density Matter (LDM) beamline has been built as part of the FERMI free-electron laser (FEL) facility to serve the atomic, molecular and cluster physics community. After the commissioning phase, it received the first external users at the end of 2012. The design and characterization of the LDM photon transport system is described, detailing the optical components of the beamline. [ABSTRACT FROM AUTHOR]

The dynamics of laser-excited sodiumatoms at the surface of heliumnanodroplets has been investigated as a function of quantum state.For all cases, excitation of the system leads to desorption of thesodium atom from the droplet surface. The mean kinetic energy of thedesorbed atoms scales linearly with excitation frequency, indicativeof an impulsive desorption process. The energy partitioning betweenthe helium and the desorbing sodium atom depends on the quantum stateand appears to be related to the size and shape of the electron orbital.The speed distributions of desorbed NaHe exciplexes point toward adirect formation process of an exciplex with no internal energy. Photoelectronspectroscopy reveals an increasing importance of helium-induced relaxationwith increasing quantum state, which is tentatively attributed tocurve crossing between different NaHeNinteraction potentials during the desorption process. [ABSTRACT FROM AUTHOR]

The dynamics following the photoexcitation of Ag atoms in 4He nanodroplets via the 5p 2P1/2← 5s 2S1/2 and 5p 2P3/2← 5s 2S1/2 transitions has been investigated in a joint experimental and theoretical effort. It has been experimentally found that upon excitation to the 2P1/2 state, the Ag atoms are ejected with a speed distribution peaking at about 55 m s−1. When Ag is excited to the 2P3/2 state, a rich phenomenology is found. While a fraction of the impurities remains solvated, the impurities that are ejected from the droplets either as Ag or AgHe have speed distributions similar, but not identical, to those found for excitation to the 2P1/2 state. The experimental findings are qualitatively analyzed within a three-dimensional, time-dependent density functional approach for the helium droplet. The dynamics of the Ag–4He1000 system has been followed for several tens of picoseconds, long enough to observe AgHe exciplex formation and the departure of the photoexcited Ag atom from the helium droplet. [ABSTRACT FROM AUTHOR]