Laser Spectroscopic Investigations Research Articles

Effect of Dehydrated Trehalose Matrix on the Kinetics of Forward Electron Transfer Reactions in Photosystem I

Abstract The effect of dehydration on the kinetics of forward electron transfer (ET) has been studied in cyanobacterial photosystem I (PS I) complexes in a trehalose glassy matrix by time-resolved optical and EPR spectroscopies in the 100 fs to 1 ms time domain. The kinetics of the flash-induced absorption changes in the subnanosecond time domain due to primary and secondary charge separation steps were monitored by pump–probe laser spectroscopy with 20-fs low-energy pump pulses centered at 720 nm. The back-reaction kinetics of P700 were measured by high-field time-resolved EPR spectroscopy and the forward kinetics of A 1A • − / A 1 B • − → F X ${\rm{A}}_{{\rm{1A}}}^{ \bullet - }/{\rm{A}}_{1{\rm{B}}}^{ \bullet - } \to {{\rm{F}}_{\rm{X}}}$ by time-resolved optical spectroscopy at 480 nm. The kinetics of the primary ET reactions to form the primary P 700 • + A 0 • − ${\rm{P}}_{700}^{ \bullet + }{\rm{A}}_0^{ \bullet - }$ and the secondary P 700 • + A 1 • − ${\rm{P}}_{700}^{ \bullet + }{\rm{A}}_1^{ \bullet - }$ ion radical pairs were not affected by dehydration in the trehalose matrix, while the yield of the P 700 • + A 1 • − ${\rm{P}}_{700}^{ \bullet + }{\rm{A}}_1^{ \bullet - }$ was decreased by ~20%. Forward ET from the phylloquinone molecules in the A 1 A • − ${\rm{A}}_{1{\rm{A}}}^{ \bullet - }$ and A 1 B • − ${\rm{A}}_{1{\rm{B}}}^{ \bullet - }$ sites to the iron–sulfur cluster FX slowed from ~220 ns and ~20 ns in solution to ~13 μs and ~80 ns, respectively. However, as shown by EPR spectroscopy, the ~15 μs kinetic phase also contains a small contribution from the recombination between A 1 B • − ${\rm{A}}_{1{\rm{B}}}^{ \bullet - }$ and P 700 • + . ${\rm{P}}_{700}^{ \bullet + }.$ These data reveal that the initial ET reactions from P700 to secondary phylloquinone acceptors in the A- and B-branches of cofactors (A1A and A1B) remain unaffected whereas ET beyond A1A and A1B is slowed or prevented by constrained protein dynamics due to the dry trehalose glass matrix.

Collisional shift and broadening of the transition lines in pionic helium

We calculate the density shift and broadening of selected dipole transition lines of pionic helium in gaseous helium at low temperatures up to T=12 K and pressure up to a few bar. In the approximation of binary collisions the shift and broadening depend linearly on the density; we evaluate the slope of this linear dependence for a few spectral lines of known experimental interest, and also investigate its temperature dependence. We find a blue shift of the resonance frequencies of the $(n,l)=(16,15) \rightarrow (16,14)$, $(17,16) \rightarrow (17,15)$, and $(16,15)\rightarrow(17,14)$ unfavored transitions, and a red shift for the favored one $(17,16) \rightarrow (16,15)$. The results are intended to significantly increase the efficiency of the laser spectroscopy investigations of pionic helium and help the interpretation of the experimental data.

Susceptibility for Doppler-broadened two-level atoms in pump–probe spectroscopy: analytical solutions revisited

We present an analytical study of susceptibility in pump–probe laser spectroscopy for an atomic medium composed of two-level atoms in the Doppler-broadened limit. We derive an accurate analytical formula for susceptibility up to the first (arbitrary) order at the Rabi frequency of the probe (pump) beam. This is expressed in a succinct single term rather than as a complicated summation, as presented in previous papers. We also derive analytical solutions for the linewidth and peak value of the absorption spectrum of the probe beam.

Variation of the Observed Widths of La I Lines with the Energy of the Upper Excited Levels, Demonstrated on Previously Unknown Energy Levels

We performed systematic laser spectroscopic investigations of La I spectral lines, using optogalvanic detection. Sixteen previously unknown even parity levels, having energies between 40,300 and 44,300 cm-1, are reported. These levels classify altogether 67 lines, not listed in spectral tables. The new levels were found due to the observation of the depopulation of the lower levels of the excited transitions. We found a remarkable variation of the observed widths of single hyperfine structure components dependent on the energy of the upper excited levels. Some levels having energies higher than 43,000 cm-1 appear to have a very high ionization probability.

Fragmentation pathways of dianionic [Pt2(μ-P2O5H2)4 + X,Y]2− (X,Y = H, K, Ag) species in an ion trap induced by collisions and UV photoexcitation

We report on the first gas phase study of dianionic species [Pt2(μ-P2O5H2)4+X,Y]2− (X,Y=H, K, Ag) derived by counterion attachment to the dimetallic [Pt2(μ-P2O5H2)4]4− complex (abbrev. Ptpop). The combined electrospray ionization mass spectrometry and laser spectroscopy investigation provides intrinsic properties and fragmentation pathways without solvent interaction in comparison to calculations by density functional theory (DFT) methods. Drastic differences between collision induced (loss of water) and UV photo-induced fragmentation (electron photodetachment or loss of Ag0) in a quadrupole ion trap are observed. The underlying fragmentation pathways are explored in detail by MSn. Upon UV photoexcitation [Ptpop+H,Ag]2− and [Ptpop+2Ag]2− exhibit dissociation of Ag0 indicating an intramolecular electron transfer, whereas mainly electron photodetachment is observed for [Ptpop+2H]2− and [Ptpop+H,K]2−. Channel specific photo-induced dissociation spectra in the range of 250–400nm reveal the main band at ∼370nm in remarkable resemblance to aqueous absorption spectra. Additional absorption features, distinct broadening and spectroscopic shifts depending on counterions and fragment channels are also observed. For the ground state structures a side-on coordination of metal counterions to the [Pt2(μ-P2O5H2)4]4− framework is deduced based on fragmentation pathways, results of geometry optimizations and calculated absorption spectra.

Nature and strength of sulfur-centred hydrogen bonds: laser spectroscopic investigations in the gas phase and quantum-chemical calculations

The importance of Sulfur centred hydrogen bonds (SCHBs) cannot be underestimated given the current day knowledge of its non-covalent interactions prevalent in many biopolymers as well as in organic systems. Based on the distance/angle constraints available from the structural database, these interactions have been interchangeably termed as van der Waals/hydrogen bonded complexes. There is a lack of sufficient spectroscopic evidence that can unequivocally term these interactions as hydrogen bonding interactions. In this review we present laser spectroscopic investigations of isolated binary complexes of H-bond donor-acceptor molecules containing Sulfur atom. The complexes were formed using supersonic jet expansion method and the IR/UV spectroscopic investigations were carried out on mass selected binary complexes. The pertinent questions regarding SCHBs addressed herein are (1) Is electronegativity the controlling factor to be a potent H-bond donor/acceptor? (2) How do SCHBs compare with their oxygen counterpart? (3) What is the nature of SCHBs, i.e. what are the dominating forces in stabilising these hydrogen bonds? (4) Do SCHBs follow classical H-bond acid–base formalism? (5) Are SCHBs found in peptides and proteins? If so, what are their strengths? Do they control the structure of the peptides? The experimental investigations were also supported by high level of ab initio computations.

Ionization potentials of the lanthanides and actinides – towards atomic spectroscopy of super-heavy elements

A study on the systematic of the atomic ionization potentials for both, the lanthanide and actinide elements have been performed. The existing experimental basis, predominantly relying on results from resonance ionization spectroscopy, has been extended by novel laser spectroscopic investigations on the elements Au, Dy, Pr and Pa. Conclusive results of suitable precision for the ionization potentials could be obtained except for Pa, due to the complexity of its atomic spectrum. Nevertheless, a consistent interpretation of the observed trends for the ionization potentials of lanthanides and actinides was attempted. The series of lanthanides depicts the two well-known, completely smooth, linear trends above and below half-shell closure, from which an expectation value for the missing ionization potential of the all radioactive element promethium of IP Pm= 44985(140) cm −1 was derived. In contrast, the lighter members of the actinide series below the half-filled shell exhibit a significant deviation from predictions, which are ascribed dominantly to relativistic influences affecting the energetic position of the multitude of low-lying configurations. With the assumption of removal of a 6d electron during the ionization process agreement between theory and experiment and a smooth, even though not linear behavior, is obtained also in this region of the Periodic Table. This new interpretation could help to better predict similar trends and systematics for elements heavier than the actinides. Particularly relevant in this respect are the super-heavy elements, which are produced only in minuscule atom numbers and thus were not accessible for any atomic physics study yet.

Resonant three-photon ionization spectroscopy of atomic Fe

Laser spectroscopic investigations on high-lying states around the ionization potential (IP) in the atomic spectrum of Fe have been carried out for the development of a practical three-step resonance ionization scheme accessible by Ti: sapphire lasers. A hot cavity laser ion source, typically used at on-line radioactive ion beam production facilities, was employed in this work. Ionization schemes employing high-lying Rydberg and autoionizing states populated by three-photon excitations were established. Five new Rydberg and autoionizing Rydberg series converging to the ground and to the first four excited states of Fe II are reported. Analyses of the Rydberg series yield the value 63 737.686 ± 0.068 cm−1 for the ionization potential of iron.

Modelling of emission spectra of Pr I by summarizing hyperfine patterns of overlapping spectral lines

We present studies of the the hyperfine (hf) structure of spectral lines of Praseodymium (Pr) by laser spectroscopic investigations as well as by analyzing Fourier Transform (FT) spectra. The experimental part of our work is done with the technique of laser-induced fluorescence (LIF) spectroscopy in a hollow cathode lamp. We present detailed studies of the very small region of 1.5 A (from 5810.5 A to 5812.0 A) in the visible area of the FT spectrum where at least 14 spectral lines are overlapping. In the investigated region we discovered two new even levels and three new odd levels and could classify 11 new spectral lines. The final goal is to model the emission spectrum by a sum of the hf profiles of all spectral lines in a certain region. Application of such modelling may be found in analyzing highly resolved stellar spectra.

Cooling and Auger Recombination of Charges in PbSe Nanorods: Crossover from Cubic to Bimolecular Decay

The cooling and Auger recombination of electron-hole pairs in PbSe quantum dots (QDs) and a series of nanorods (NRs) with similar diameter and varying length was studied by ultrafast pump-probe laser spectroscopy. Hot exciton cooling rates are found to be independent of nanocrystal shape. The energy relaxation rate decreases during cooling of charges, due to reduction of the density of electronic states. Auger recombination occurs via cubic third-order kinetics of uncorrelated charges in the QDs and NRs with length up to 29 nm. On increasing the NR length to 52 nm, a crossover to bimolecular exciton decay is found. This suggests a spatial extent of the one-dimensional exciton of 30-50 nm, which is significantly smaller than the value of 92 nm for the three-dimensional exciton diameter in bulk PbSe. The Auger decay time increases with NR length, which is beneficial for applications in nanocrystal lasers as well as for generation of free charges in photovoltaics.

Cold highly charged ions in a cryogenic Paul trap

Narrow optical transitions in highly charged ions (HCIs) are of particular interest for metrology and fundamental physics, exploiting the high sensitivity of HCIs to new physics. The highest sensitivity for a changing fine structure constant ever predicted for a stable atomic system is found in Ir17 + . However, laser spectroscopy of HCIs is hindered by the large (∼ 106 K) temperatures at which they are produced and trapped. An unprecedented improvement in such laser spectroscopy can be obtained when HCIs are cooled down to the mK range in a linear Paul trap. We have developed a cryogenic linear Paul trap in which HCIs will be sympathetically cooled by 9Be + ions. Optimized optical access for laser light is provided while maintaining excellent UHV conditions. The Paul trap will be connected to an electron beam ion trap (EBIT) which is able to produce a wide range of HCIs. This EBIT will also provide the first experimental input needed for the determination of the transition energies in Ir17 + , enabling further laser-spectroscopic investigations of this promising HCI.

Precision atomic physics techniques for nuclear physics with radioactive beams

Atomic physics techniques for the determination of ground-state properties of radioactive isotopes are very sensitive and provide accurate masses, binding energies, Q-values, charge radii, spins and electromagnetic moments. Many fields in nuclear physics benefit from these highly accurate numbers. They give insight into details of the nuclear structure for a better understanding of the underlying effective interactions, provide important input for studies of fundamental symmetries in physics, and help to understand the nucleosynthesis processes that are responsible for the observed chemical abundances in the Universe. Penning-trap and storage-ring mass spectrometry as well as laser spectroscopy of radioactive nuclei have now been used for a long time but significant progress has been achieved in these fields within the last decade. The basic principles of laser spectroscopic investigations, Penning-trap and storage-ring mass measurements of short-lived nuclei are summarized and selected physics results are discussed.

Laser Spectroscopic Investigations of Praseodymium I Transitions: New Energy Levels

We report the discovery of about 140 new energy levels of the neutral praseodymium atom, found by means of laser-induced fluorescence spectroscopy. Their energy has been determined with an uncertainty of 0.010 cm−1 using a wave number calibrated Fourier-transform spectrum.

Investigation of Relaxation Processes in Nanocomposites by Transient Grating Experiments

We report a comparative study on the photo-elastic and photo-thermal features of two nanocomposites based on MoS2 nanotubes dispersed into two different polyurea elastomeric matrices, which have different crosslinking properties. The time-resolved laser spectroscopic investigation on these nanocomposites by transient grating (TG) experiments has been performed. The relaxation kinetics on a very wide time scale has been measured from nanosecond to hundred of microseconds. The TG data on both pure polymers shows fast acoustic oscillations and a slow thermal decay. No structural relaxation is appearing in the experimental time window. Surprisingly, the presence of nanotubes in both the polymeric matrixes induces intermediate dynamics clearly detected by the TG signal. The features of this unexpected dynamic features recall the structural relaxation typical of glass-formers but its origin is still unclear.

Resonance ionization spectroscopy of thorium isotopes–towards a laser spectroscopic identification of the low-lying 7.6 eV isomer of 229Th

In-source resonance ionization spectroscopy was used to identify an efficient and selective three-step excitation/ionization scheme of thorium, suitable for titanium:sapphire (Ti:sa) lasers. The measurements were carried out in the preparation of laser spectroscopic investigations for an identification of the low-lying 229mTh isomer predicted at 7.6 ± 0.5 eV above the nuclear ground state. Using a sample of 232Th, a multitude of optical transitions leading to over 20 previously unknown intermediate states of even parity as well as numerous high-lying odd parity auto-ionizing (AI) states were identified. Level energies were determined with an accuracy of 0.06 cm−1 for intermediate and 0.15 cm−1 for AI states. Using different excitation pathways, an assignment of total angular momenta for several energy levels was possible. One particularly efficient ionization scheme of thorium, exhibiting saturation in all three optical transitions, was studied in detail. For all three levels in this scheme, the isotope shifts of the isotopes 228Th, 229Th and 230Th relative to 232Th were measured. An overall efficiency including ionization, transport and detection of 0.6% was determined, which was predominantly limited by the transmission of the mass spectrometer ion optics.

Investigation of the hyperfine structure of Pr I and Pr II lines based on highly resolved Fourier transform spectra

We report the recording of new highly resolved Fourier transform spectra of the neutral praseodymium atom. With the help of the new spectra we found about 9000 new lines from which—in the region 3260 to 9880 Å—1194 could be classified as transitions between energy levels of the Pr atom and 19 as transitions of the Pr ion. Twenty-three new atomic energy levels of odd parity and one of even parity were discovered during this first examination. The spectra might be very helpful for further laser spectroscopic investigations of the hyperfine structure of Pr I and Pr II transitions.

Spectral shifts and structures of phenol⋯Arn clusters

A laser spectroscopic investigation of phenol...Ar(n) (n = 1-6) clusters in the first electronically excited state (S(1)) and the cationic ground state (D(0)) is reported. Resonance enhanced two-photon ionisation (R2PI) spectra have been recorded for the investigation of the S(1) state. The origins of S(1)← S(0) (S(1)0(0)) transition of phenol...Ar(n) (n = 1, 2,4-6) are all red shifted compared to the S(1)0(0) state of the monomer by 33 cm(-1), 67 cm(-1), 10 cm(-1), 20 cm(-1), 44 cm(-1), respectively. However, the origin of the phenolAr(3) cluster is blue shifted by 25 cm(-1). For the investigation of the ionic ground state photoionization efficiency (PIE) and mass-analyzed-threshold ionization (MATI) spectroscopy have been applied. The spectra of phenol...Ar(3) and phenol...Ar(4) yield values for the ionization energy (IE) of 68,077 ± 15 cm(-1) and 67,948 ± 15 cm(-1). With the combination of theoretical methods and R2PI, PIE and MATI spectroscopy, the major species present have been positively identified.

Time-Resolved Picosecond Photocurrents in Contacted Carbon Nanotubes

We introduce coplanar stripline circuits to resolve the ultrafast photocurrent dynamics of freely suspended carbon nanotubes (CNTs) in the time domain. By applying an on-chip pump-probe laser spectroscopy, we demonstrate that CNTs, contacted by metal electrodes, exhibit a picosecond photocurrent response. We find a combination of an optically induced ultrafast displacement current, transport of photogenerated charge carriers at the Fermi velocity to the electrodes, and interband charge-carrier recombination processes to dominate the ultrafast photocurrent of the CNTs.

Infrared diode laser spectroscopic investigation of four C–H stretching vibrational modes of propylene oxide

The high resolution ro-vibrational spectra of propylene oxide have been recorded for the first time in the C–H stretching region using a rapid scan infrared laser spectrometer equipped with an astigmatic multipass cell. Detailed analyses of the three C–H bands centered at 2942 and 2975 cm −1 reveal that there are significant perturbations in the methyl C–H symmetric and the two near-degenerate asymmetric fundamental bands. Low resolution Fourier transform infrared spectra of propylene oxide have also been measured with a static gas sample and under supersonic jet conditions. The MP2/6-311++G(d,p) harmonic and anharmonic frequency calculations have been performed to aid the C–H stretching band assignment.

Production and investigations of negative osmium ions for fundamental applications: REOSTRAP

High quality measurements in respect to accuracy, resolving power and sensitivity using negative osmium ions confined in ion traps will contribute to answer questions in modern fundamental physics. A proposed system to carry out these measurements would require a laser desorption ion source, and an ion-trap system. Following the recent laser spectroscopy investigations at the Max-Planck Institute for Nuclear Physics in Heidelberg, the goals of the proposed system should focuss on the laser cooling of negative osmium ions since such a system could be used to cool antiprotons to very low temperatures via collisions (sympathetic cooling) for efficient antihydrogen formation in its ground state. Furthermore, together with rhenium ions, the confinement of osmium ions in a Penning trap is important to determine the mass difference 187Re-187Os, and therefore the Q-value in the decay of 187Re (T 1/2 = 4×1010 years) with unprecedented accuracy. This Q-value is an important constraint for the determination of the mass of the electron antineutrino as aimed by the international MARE collaboration. In this paper several mechanisms are considered for the preparation of the negative ions in order to apply laser cooling.