Following excited-state chemical shifts in molecular ultrafast x-ray photoelectron spectroscopy

The conversion of photon energy into other energetic forms in molecules is accompanied by charge moving on ultrafast timescales. We directly observe the charge motion at a specific site in an electronically excited molecule using time-resolved x-ray photoelectron spectroscopy (TR-XPS). We extend the concept of static chemical shift from conventional XPS by the excited-state chemical shift (ESCS), which is connected to the charge in the framework of a potential model. This allows us to invert TR-XPS spectra to the dynamic charge at a specific atom. We demonstrate the power of TR-XPS by using sulphur 2p-core-electron-emission probing to study the UV-excited dynamics of 2-thiouracil. The method allows us to discover that a major part of the population relaxes to the molecular ground state within 220–250 fs. In addition, a 250-fs oscillation, visible in the kinetic energy of the TR-XPS, reveals a coherent exchange of population among electronic states.

Evaluation and Comparison of The Current-Voltage (I-V) Performance for Both Silicon Solar Cells and Dye Sensitized Solar Cell (DSSC) Covered with Natural Plant Dyes

A new concept based on introducing natural dye-sensitized molecules on the surface of Silicon Si solar cell namely “dyed Si solar cell” is introduced. This dye/Si interface is thought to be effectively enhanced efficiency. The IV readings are compared among (a) blank and covered Si solar cells, (b) DSSC using the same sensitized molecules. The results were recorded with different physical parameters like UV-visible Spectrum dyes, light intensity, cell area, and different fabrication Also, cell stability has been recorded. These results serve simply to give some of the cutting-edge of dyed Si solar cell with a huge improvement in its efficiency up to 121% with pot marigold flower dye (CC) dye at its optimum case and 16.53% in arthropodafotos-de- flower dye (ZZ) dye at its lowest case. While in DSSC, the efficiencies associated with the same natural dyes were very limited, rather sometimes they get lower. The results have been compared with similar group studies. Our new concept may be used as a highly promising technology for the dyed Si solar cell to give higher efficiency compare with its blank Si solar cell due to the suitability of dyes with silicon semiconductor, we suggest a figure for the new cell which is an ambiguous mechanism of cooperation between excited molecule with the promoted electron of silicon semiconductor, Si.

Линейный дихроизм и двулучепреломление зондирующего излучения в спектроскопии накачка-зондирование в многоатомных молекулах

Probe beam dichroism and birefringency occurring in the excited states of polyatomic molecules under excitation with two femtosecond laser pulses have been theoreticaly studied as a function of delay between the pulses. General expressions describing the change of intensity and polarization of the probe laser pulse passed through the solution of arbitrary polyatomic molecules at any initital polarization of the laser pulses have been derived using the spherical tensor approach. The expressions take into account the coherence in excited molecule vibrational states and decay of these states due to vibrational relaxation, rotational diffusion, and radiative transitions. The expressions describe the effects of probe beam linear dichroism and birefringency occurring in molecular excited states. As shown, under certain conditions both effects can be observed simultaneously. It has been concluded that within the geometry of almost collinear propagation of the pump and probe laser pulses through the molecular sample the contributions from linear dichroism and birefringency can be completely separated from each other by an appropriate choice of the polarization analyzer placed in front of the photodetector. The obtained expressions were applied for description of the signals that can be recorded experimentally by means of the polarization-modulation scheme developed in the recent author's publication (Gorbunova et al,Phys. Chem. Chem. Phys. 2020, Vol. 22, 18155−18168). The theory predicts that the modulated signals of dichroism and birefringency appear in quadrature with respect to the double frequency reference modulation signal.

Charge transfer and electromagnetic enhancement processes revealed in the SERS and TERS of a CoPc thin film

Phthalocyanines are frequently used as probing molecules in the field of single-molecule surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS). In this work, we systematically compare the SERS and TERS spectra from a thin cobalt phthalocyanine (CoPc) film that is deposited on a Au film. The contributions from electromagnetic (EM), resonance, and charge-transfer enhancements are discussed. Radially and azimuthally polarized vector beams are used to investigate the influences of molecular orientation and the localized surface plasmon resonance (SPR). Furthermore, two different excitation wavelengths (636 and 532 nm) are used to study the resonant excitation effect as well as the involvement of the charge-transfer processes between CoPc and the Au substrate. It is shown that the Raman peaks of CoPc are mostly enhanced by 636 nm excitation through a combination of resonant excitation, high EM enhancement, and chemical enhancement via charge transfer from the metal to the molecule. At 532 nm excitation, however, the SERS and TERS spectra are dominated by photoluminescence, which originates from a photo-induced charge-transfer process from the optically excited molecule to the metal. The contributions of the different enhancement mechanisms explain the optical contrasts seen in the TERS images of Au nanodisks covered by the CoPc film. The insight achieved in this work will help to understand the optical contrast in sub- or single-molecule TERS imaging and apply SERS or TERS in the field of photocatalysis.

Unconventional Solar Energy: Singlet Fission

Described simply, singlet fission is a process in which a singlet excited molecule transfers about half of its excitation energy to a neighbor molecule, both end up in their triplet state, and the two triplet excitations diffuse apart. The process is of interest for solar cells. Used in conjunction with ordinary solar cell material, a layer of singlet-fission material offers an opportunity to utilize higher energy photons more efficiently. The maximum theoretical efficiency is then close to 1/2 instead of the Shockley-Queisser value of 1/3 that applies to an ordinary single-junction cell. The problem that prevents an immediate production of singlet fission solar cells is the dearth of sufficiently stable efficient materials. The formulation of simple rules for the design of suitable compounds for the purpose is discussed.

Time-dependent view of an isotope effect in electron-nuclear nonequilibrium dynamics with applications to N2

Isotopic fractionation in the photodissociation of N2 could explain the considerable variation in the 14N/15N ratio in different regions of our galaxy. We previously proposed that such an isotope effect is due to coupling of photoexcited bound valence and Rydberg electronic states in the frequency range where there is strong state mixing. We here identify features of the role of the mass in the dynamics through a time-dependent quantum-mechanical simulation. The photoexcitation of N2 is by an ultrashort pulse so that the process has a sharply defined origin in time and so that we can monitor the isolated molecule dynamics in time. An ultrafast pulse is necessarily broad in frequency and spans several excited electronic states. Each excited molecule is therefore not in a given electronic state but in a superposition state. A short time after excitation, there is a fairly sharp onset of a mass-dependent large population transfer when wave packets on two different electronic states in the same molecule overlap. This coherent overlap of the wave packets on different electronic states in the region of strong coupling allows an effective transfer of population that is very mass dependent. The extent of the transfer depends on the product of the populations on the two different electronic states and on their relative phase. It is as if two molecules collide but the process occurs within one molecule, a molecule that is simultaneously in both states. An analytical toy model recovers the (strong) mass and energy dependence.