Orientation, rotation and solvation of ions in helium nanodroplets

Helium nanodroplets are liquid, finite size, nanoscale helium clusters that have been employed since the 1990s as a matrix for high-resolution spectroscopy of molecules. Spectroscopic studies of ionic species inside helium droplets were not however realised until 2010, even though the potential information about their structure is of special interest since they are fundamental reaction intermediates in biochemistry. Additionally, the interaction of these species with the helium environment can be determined from these experiments, which is interesting from a fundamental point of view.

To determine structural information and study product selectivity, we proposed using high electric fields to orient the molecular ions in helium nanodroplets. Pendular state spectroscopy of neutral polar molecules in helium nanodroplets in the presence of electric fields has previously given satisfactory results to achieve molecular orientation, allowing one to distinguish biomolecular isomers. Our first experiments validated the technique for neutral aniline since the polarisation difference ratios, which express the degree of orientation, obtained from the NH2-stretch transitions were well-reproduced by calculations. On the other hand, the measured degree of orientation of cationic aniline is noticeably smaller than calculations show. Even so, the variation of the degree of orientation with electric field lets us conclude that orientation of the ions in helium droplets was observed. The results of several experiments performed to investigate the possible effects influencing the apparent orientation of the cations did not lead to any straightforward explanation. Our study therefore shows that orientation of molecular ions indeed seems possible and leads the way to possible future experiments.

To examine the formation of ionic-alkali solvation complexes in helium droplets upon photoionisation of the alkali, sodium, and understand the helium-dopant interaction we have performed ion spectroscopy. We found that several scenarios are possible after ionisation including dissociation, solvation and complete solvent evaporation depending on the ionisation process, droplet size and number of water ligands considered. Direct ionisation of sodium leads to solvation in the droplet, collision with the water molecules and formation of the ionic complexes if the droplet size is sufficiently large. For these conditions, rovibrational spectroscopy of Na(OH2)+ was performed to obtain the A rotational constant in helium droplets and thus determine if the dopant-helium interaction affects the moment of inertia. From the results, we could confirm the latter but could not deduce the extent to which this was the case. This study shows how the spectral information of the solvated ions is useful to understand complexation dynamics in helium droplets and acquire further understanding about the dopant-helium interaction.

Steric Effects in the Chemisorption of Vibrationally Excited Methane on Nickel

Bruce Yoder

In this thesis, steric effects in the dissociative chemisorption of quantum state-prepared methane on single crystal surfaces of nickel (Ni(100) and Ni(110)) are detected and quantified for the first time. Exploiting a new, continuous-wave, high-power, single-mode infrared optical parametric oscillator, I produced an intense, quantum state-prepared molecular beam by rapid adiabatic passage. During the infrared excitation of the antisymmetric (ν3) stretch of CH4 or the C-H (ν1) stretch of CD3H by linearly polarized radiation, the angular momentum and vibrational transition dipole moment of methane is aligned in the laboratory frame. The excited, aligned molecular beam is used to probe the stereodynamics of the chemisorption reaction of vibrationally excited methane. For the reaction on Ni(100), an increase in state-prepared methane reactivity of nearly 60% is observed when the laser polarization direction is changed from normal to parallel to the surface. The dependence of the alignment effect on the rotational branch used for excitation (P-, Q-, or R-branch) indicates that alignment of the vibrational dipole moment rather than the angular momentum is responsible for the alignment dependent reactivity. Dephasing of the initially prepared alignment due to hyperfine coupling is observed to be on the timescale of 5-15 microseconds which does not preclude the study of alignment dependent reactivity in our experimental setup. Reactivity decreased monotonically from parallel to perpendicular alignment for both methane isotopologues studied. The alignment effect is shown to be independent of incident velocity for CH4(ν3) and decreases with increasing velocity of CD3H(ν1). For the Ni(110) surface, which consists of parallel rows of closely spaced Ni atoms separated by one-layer deep troughs, I probed if the reactivity depends on the methane alignment relative to the direction of the surface rows. The CH4(ν3) reactivity increases a factor of two when the laser polarization direction is changed from normal to parallel to the surface. Alignment of the vibration perpendicular to the surface rows produced a ∼10% higher reactivity than alignment parallel to the surface rows. Steric effects in a chemical reaction reveal detailed information about the reactive potential energy surface, which makes experimental studies of stereochemistry a powerful probe of microscopic chemical dynamics. The results in this thesis demonstrate and quantify specific steric requirements for this benchmark gas-surface reaction and will serve as a stringent test of multi-dimensional dynamics calculations.

EPFL2010

Ultrafast Visible and Ultraviolet Fluorescence Studies of Molecular and Biological Sytems in Solution

Olivier Christian Bräm

A wide variety of physical and chemical processes at the molecular level, as charge or energy transfer, solvation, electronic as well as vibrational relaxation, is at the origin of the biological functionality of proteins. The work reported in this thesis is devoted to the study of these molecular dynamics in different chromophore systems. More specifically, this study is focused on the role of these dynamics in the ultrafast photophysics and photochemistry of haemoproteins. This important class of proteins has been widely studied through various spectroscopic techniques, in order to understand their functionality. These proteins contain several chromophore moieties, among which one aminoacid residue, Tryptophan, and the haem prosthetic group, which are of particular interest in this work. While Tryptophan is used as a probe of the local environment via intermolecular relaxation dynamics, we investigated the role of haem intramolecular dynamics on the protein functionality. While different kind of ultrafast spectroscopic techniques are indicated to tackle these issues, like pump probe or photon-echo peak shift experiments, we choose a different approach to probe directly the energetic relaxation, which consists in following the ultrafast changes in the photoluminescence spectrum. The dynamics are then extracted from the spectral evolution of the emission. With this purpose, we implemented a fluorescence up-conversion set-up, which allows us resolving temporally and spectrally the photoluminescence of the sample under investigation, excited with an ultrashort light pulse, and with a temporal resolution on the femtosecond timescale. The thesis is structured in 9 chapters. Chapter 1 resumes the theoretical background required for the interpretation of the experimental results. Chapter 2 describes the photophysical properties of the different systems investigated. Chapter 3 presents the experimental technique of fluorescence up-conversion, used to achieve broadband femtosecond detection of photoluminescence. In Chapter 4, we present a study on the role of ultrafast vibrational and structural dynamics on the time-resolved fluorescence spectra from two UV dyes. The measurements allow us exploring the capabilities of the set-up in revealing subtle relaxation dynamics with an unequaled accuracy. In Chapter 5, we move on to the study of the relaxation dynamics of Tryptophan (Trp) in water. This aminoacid served as an in vivo probe of solvation dynamics, owing to its high dipolar moment in its excited state. Our study focuses on water reorganization around the chromophore and on the distinction of this important process from the electronic internal conversion occurring on a similar timescale. In the framework of the study of protein dynamics, Chapter 6 reports the investigation of a family of molecules extensively found in nature, namely porphyrins. In these molecular systems, a quite complex pattern of intramolecular relaxation mechanisms occurs, which needed to be clarified. A systematic study on the free base and on a wide series of metallo-porphyrins definitely clarified the picture of their electronic relaxation pathways. We finally elaborate a comprehensive scheme of this relaxation, including substituent's influence. The mechanisms observed are intrinsically related to the photophysical properties used by nature, as will be illustrated in the next Chapter. Trp and metallo-porphyrin constitute the key natural chromophore molecules for the non-invasive spectroscopic investigation of complex biological systems as haemoproteins. In Chapter 7, we follow relaxation dynamics of photo-excited Tryptophan and Haem (Fe-porphyrin) in ferro- and ferricytochrome c and in Myoglobin in its met form. In the former protein, an ultrafast energy transfer from Tryptophan to Heme is evidenced, the efficiency of which is found to depend on the oxidation state. The energetic relaxation pathway of the haem, also dependent on the oxidation state, is characterized by a porphyrin to metal charge transfer that triggers the ligand dissociation. In metMyoglobin, the fluorescence contribution of the two Tryptophans (W14 and W7), also quenched by resonant energy transfer to the haem, are temporally and spectrally separated, and the solvation dynamics of W7, located in the water-protein interface, is followed. The haem of metMb, which is in the ferric form, shows an electronic relaxation similar to that of ferricytochrome c. In parallel to the biology oriented investigations presented above, we extended our study to the metal-polypiridine complexes. Due to their specific photophysics, mainly characterized by a photo-induced Metal-to-Ligand Charge Transfer (MLCT), they are extensively used in photochemical applications involving charge transfer dynamics. In particular, they are used as sensitizer in the Dye-Sensitized-Solar-Cells (DSSCs). In Chapter 8, we present studies of these dyes, both in solution and adsorbed on a substrate, in order to understand the role of intra- and intermolecular dynamics on the electron injection process occurring upon absorption of light. Finally, Chapter 9 summarizes the conclusions of the investigations of the various systems studied.

EPFL2011

A Combination of Cold Ion Spectroscopy and Ion Mobility for the Study of Complex Peptides and Small Proteins

Georgios Papadopoulos

We present in this work experiments that push the limits of cold ion spectroscopy to the study of complex peptides and small proteins in the gas phase. Although the low temperature attainable in a cold ion trap greatly simplifies the electronic spectra of large molecules, conformational heterogeneity can still be a significant source of congestion, complicating spectroscopic analysis. To overcome this hurdle, we propose an in tandem combination of ion mobility spectrometry and photofragment spectroscopy. This permits for conformational separation prior to laser interrogation. Towards this end, we describe a proof-of-principle experiment where we combine high- Field Asymmetric waveform Ion Mobility Spectrometry (FAIMS) with electronic spectroscopy. We demonstrate that using FAIMS to separate gas-phase peptide conformers before injecting them into a cold ion trap allows one to decompose a dense spectrum into contributions from different conformational families. In the inverse sense, cold ion spectroscopy can be used as a conformation-specific detector for ion mobility, allowing one to separate an unresolved mobility peak into contributions from different conformational families. The doubly protonated peptide bradykinin serves as a good test case for the marriage of these two techniques as it exhibits a considerable degree of conformational heterogeneity that results in a highly congested electronic spectrum. Our results demonstrate the feasibility and advantages of directly coupling ion mobility with spectroscopy and provide a diagnostic of conformational isomerization of this peptide after being produced in the gas phase by electrospray. In a second series of studies we show the potential but also the limitations of our spectroscopic approach for investigating small, naturally occurring proteins, by applying it to ubiquitin, a protein of 76 amino acids. We present electronic photofragment spectra of various protonation states of ubiquitin and we compare our results to literature data acquired by ion mobility spectrometry. These experiments show that the information obtained by laser interrogation of molecules of this size is limited and could be enhanced by a combination with ion mobility techniques. Although some of the results obtained may be important in characterizing small proteins in the gas phase, these experiments are at an early stage and the application of photofragment spectroscopy on proteins needs more investigation.

EPFL2012