44 research outputs found
Strongly aligned molecules inside helium droplets in the near-adiabatic regime
Iodine (I) molecules embedded in He nanodroplets are aligned by a 160 ps
long laser pulse. The highest degree of alignment, occurring at the peak of the
pulse and quantified by , is measured as a
function of the laser intensity. The results are well described by calculated for a gas of isolated molecules each
with an effective rotational constant of 0.6 times the gas-phase value, and at
a temperature of 0.4 K. Theoretical analysis using the angulon quasiparticle to
describe rotating molecules in superfluid helium rationalizes why the alignment
mechanism is similar to that of isolated molecules with an effective rotational
constant. A major advantage of molecules in He droplets is that their 0.4 K
temperature leads to stronger alignment than what can generally be achieved for
gas phase molecules -- here demonstrated by a direct comparison of the droplet
results to measurements on a 1 K supersonic beam of isolated molecules.
This point is further illustrated for more complex system by measurements on
1,4-diiodobenzene and 1,4-dibromobenzene. For all three molecular species
studied the highest values of achieved in
He droplets exceed 0.96.Comment: 11 pages, 8 figure
Probing ultrafast C-Br bond fission in the UV photochemistry of bromoform with core-to-valence transient absorption spectroscopy.
UV pump-extreme UV (XUV) probe femtosecond transient absorption spectroscopy is used to study the 268 nm induced photodissociation dynamics of bromoform (CHBr3). Core-to-valence transitions at the Br(3d) absorption edge (∼70 eV) provide an atomic scale perspective of the reaction, sensitive to changes in the local valence electronic structure, with ultrafast time resolution. The XUV spectra track how the singly occupied molecular orbitals of transient electronic states develop throughout the C-Br bond fission, eventually forming radical Br and CHBr2 products. Complementary ab initio calculations of XUV spectral fingerprints are performed for transient atomic arrangements obtained from sampling excited-state molecular dynamics simulations. C-Br fission along an approximately CS symmetrical reaction pathway leads to a continuous change of electronic orbital characters and atomic arrangements. Two timescales dominate changes in the transient absorption spectra, reflecting the different characteristic motions of the light C and H atoms and the heavy Br atoms. Within the first 40 fs, distortion from C3v symmetry to form a quasiplanar CHBr2 by the displacement of the (light) CH moiety causes significant changes to the valence electronic structure. Displacement of the (heavy) Br atoms is delayed and requires up to ∼300 fs to form separate Br + CHBr2 products. We demonstrate that transitions between the valence-excited (initial) and valence + core-excited (final) state electronic configurations produced by XUV absorption are sensitive to the localization of valence orbitals during bond fission. The change in valence electron-core hole interaction provides a physical explanation for spectral shifts during the process of bond cleavage
Excited rotational states of molecules in a superfluid
We combine experimental and theoretical approaches to explore excited
rotational states of molecules embedded in helium nanodroplets using CS and
I as examples. Laser-induced nonadiabatic molecular alignment is employed
to measure spectral lines for rotational states extending beyond those
initially populated at the 0.37 K droplet temperature. We construct a simple
quantum mechanical model, based on a linear rotor coupled to a single-mode
bosonic bath, to determine the rotational energy structure in its entirety. The
calculated and measured spectral lines are in good agreement. We show that the
effect of the surrounding superfluid on molecular rotation can be rationalized
by a single quantity -- the angular momentum, transferred from the molecule to
the droplet.Comment: 5 pages, 4 figures; 5 pages, 3 figure
Rotational coherence spectroscopy of molecules in helium nanodroplets: Reconciling the time and the frequency domains
Alignment of OCS, CS and I molecules embedded in helium nanodroplets
is measured as a function of time following rotational excitation by a
non-resonant, comparatively weak ps laser pulse. The distinct peaks in the
power spectra, obtained by Fourier analysis, are used to determine the
rotational, B, and centrifugal distortion, D, constants. For OCS, B and D match
the values known from IR spectroscopy. For CS and I, they are the first
experimental results reported. The alignment dynamics calculated from the
gas-phase rotational Schr\"{o}dinger equation, using the experimental
in-droplet B and D values, agree in detail with the measurement for all three
molecules. The rotational spectroscopy technique for molecules in helium
droplets introduced here should apply to a range of molecules and complexes.Comment: ASC and LC contributed equally. 7 pages, 3 figure
Photoelectron angular distributions from resonant two-photon ionization of adiabatically aligned naphthalene and aniline molecules
Photoelectron images have been measured following the ionisation of aligned distributions of gas phase naphthalene and aniline molecules. Alignment in the adiabatic regime was achieved by interaction with a 100 ps infrared laser pulse, with ionisation achieved in a two-photon resonant scheme using a low intensity UV pulse of ∼6 ps duration. The resulting images are found to exhibit anisotropy that increases when the alignment pulse is present, with the aniline PADs peaking along the polarisation vector of the ionising light and the naphthalene PADs developing a characteristic four-lobed structure. Photoelectron angular distributions (PADs) that result from the ionisation of unaligned and fully aligned distributions of molecules are calculated using the ePolyScat ab initio suite and converted into two-dimensional photoelectron images. In the case of naphthalene excellent agreement is observed between experiment and the simulation for the fully aligned distribution, showing that the alignment step allows us to probe the molecular frame, but in the case of aniline it is clear that additional processes occur following the one-photon resonant step
Resonant multiphoton ionisation probe of the photodissociation dynamics of ammonia
The dissociation dynamics of the Ă-state of ammonia have been studied using a resonant multiphoton ionisation probe in a photoelectron spectroscopy experiment. The use of a resonant intermediate in the multiphoton ionisation process changes the ionisation propensity, allowing access to different ion states when compared with equivalent single photon ionisation experiments. Ionisation through the E′ 1A′1 Rydberg intermediate increases the observation window allowing us to monitor the excited state population for several hundred femtoseconds. The vibrational states in the photoelectron spectrum show two distinct timescales, 200 fs and 320 fs, that we assign to the non-adiabatic and adiabatic dissociation processes respectively. The different timescales derive from differences in the wavepacket trajectories for the two dissociation pathways that resonantly excite different vibrational states in the intermediate Rydberg state. The timescales are similar to those obtained from time resolved ion yield measurements, suggesting we can measure the different trajectories taken out to the region of conical intersectio
Probing nonadiabatic dynamics in isolated molecules with ultrafast velocity map imaging
Two complementary experiments were used to study the ultrafast dynamics of large molecules in the gas phase. Both experiments used time-resolved pump-probe velocity map imaging to monitor energetically dispersed spectra of isolated systems on a femtosecond timescale. A ‘bottom-up’ methodology is applied, whereby initially simple, small, systems are studied in a high level of detail, and then the complexity of system studied is gradually increased. The overall goal was to explore the concept of photostability, the mechanism whereby molecules can withstand bombardment by visible and ultraviolet light, especially in biomolecules.
In the first set of experiments, based in Warwick University, the dissociation of hydrogen atoms from neutral phenol and 2-hydroxy phenol (catechol) following
ultraviolet excitation was measured with femtosecond resolution. These experiments give unprecedented insight into the electronic structure of phenolic systems, and in particular hydrogen atom tunnelling underneath a conical intersection was directly observed. Varying the excitation energy allowed the transition from tunnelling dynamics to direct dissociation dynamics to be observed.
The second set of experiments, completed at Durham University, performed timeresolved photoelectron spectroscopy on large gaseous anions from an electrospray source. The electrospray technique allows very large ions to be introduced into the gas phase, so the bottom-up methodology can be continued. First, the dynamics of the common dye indigo carmine were explored, demonstrating that excited state proton transfer accounts for its photostability. Secondly, the dynamics of the nucleotides making up DNA were explored. The dynamics of nucleobase chromophores were shown to map onto larger nucleotide and oligonucleotide systems, retaining ultrafast photostable properties.
Finally, a new instrument was designed and constructed in Warwick. This machine will use laser desorption techniques to help further extend the size range isolated dynamics can be studied in, this time for neutrals. Overall, the bottom-up methodology grants insight into the excited state dynamics of real-world relevant molecules, at an extremely high level of detail