100 years ago, the first time scientists have directly observed how the light excites electrons, opened a chemical reaction!
Comments: light-driven reaction is the core of human vision, photosynthesis and solar power. This is the first time scientists have seen how an electron cloud expands under the action of light. This movement is a prelude to the nucleus in the molecule, and the formation of nuclei motion causes bond breakage. So study the inner workings of chemistry in detail, opens up new possibilities for understanding and controlling chemical reactions.
is the first step of all the excitation light photophysical and photochemical processes, including the main event PV, photosynthesis, light emitting diodes, photodynamic therapy, photocatalytic and human vision. This is the first step leading to changes in the electron density, and then start dynamics, and ultimately determine the outcome of the reaction. Currently, however, the nature of the excited state only indirectly inferred from the spectral measurements in the state transition out. In terms of x-ray scattering, the excited state is determined mainly by the second expression, such as the transition dipole moment of the molecule to its preferred arrangement of the intermediate product or the excited state molecular geometry changes. Interestingly, recent studies have shown that x-ray scattering, in order to reproduce the correct movement of the coherent oscillation excited molecules, the theoretical electron density correction must be included in the data analysis explained. However, larger changes in molecular structure to cover up the details of the electronic structure changes. Recently, Peter M. Weber Brown University and the University of Edinburgh, UK Adam Kirrander in \”Nature Communications\” published an article entitled \”Observation of the molecular response to light upon photoexcitation\” the article reported that 1,3-cyclohexadiene ( direct measurement CHD) molecules in the initial density of photoexcited electrons weight distribution. They use a strong light source a coherent linear accelerator (the LCLS), the hard ultrashort x-ray pulses by ultrafast x-ray scattering to depict the change in electron density. Excited electronic state properties with good spatial resolution, is consistent with theoretical predictions. Thus, the excited state electron density distribution direct experimental observation. Adam Kirrander said: \”Using X-ray scattering to determine the structure of the material more than 100 years of history, but this is the first method to directly observe the excited states of electrons.\” Photo REVIEW
Direct measurement of
Experimental molecular 1,3-cyclohexadiene (CHD) is a low-pressure gas at room temperature (FIG. 1) 1. The electron density changes. When the excitation laser 267 with a valence 1B nm, it undergoes a rapid ring opening reaction, may be captured by and ultrafast x-ray scattering, and electron spectroscopy and x-ray photoelectron spectroscopy. In this study, they used 200 nm pump pulse of higher energy to excite molecules 3p electron Rydberg states. Its advantages are: First, the electronic states 3p life is relatively long, approximately 200 fs. Second, the initial small changes in the molecular geometry which ensures that these changes do not obscure the observation signal redistribution of electrons. Third, the nature of the excitation diffusion 3p orbitals and the highest occupied molecular orbital (HOMO) significantly different, which provide additional confidence to the home electronic states in the scattering signal.
In the time-resolved x-ray scattering experiment, the average energy of the coherent light source (the LCLS) generated by a linear accelerator is 9.5 keV x-ray photons consisting of a set of probe molecules CHD. 2.3 megapixels scattering signal Cornell-SLAC pixel array detector (CSPAD) detection, and grouped according to the delay time between the x-ray probe and pump laser pulse. The image detector is decomposed into isotropic and anisotropic components. Wherein the rotating isotropic average component carries information of the electronic structure and molecular nuclear structure. In Figure 2a, they show tight binding transition from the ground state to direct evidence diffusion 3p electronic states. Differential radial distribution function ΔRDF (r) from the early 25 fs pump – probe delay time of the differential signal obtained by experiments sine transform. It describes the probability of real space distance between the front and rear electronic excitation light distribution difference. With the tight-binding molecules excited from the ground state to excited 3p diffusion state, electron density electronic short distance decreases, the electron density of the electron long distances, to verify the diffusion characteristics 3p excited state. In Figure 2b, the difference in fractional form shows the experimental signal delay time in the 25 fs, i.e., ΔS (q). When (0.3-1.6 and 1.7-2.5Å-1) is increased by analyzing the two regions q specific scattering signal, they are found to occur in a small area rapid onset q, wherein 3p electronic states characteristic strongly, then the change to be expected in the molecular structureSlow scattering signal zone in large q. As an independent comparison, Figure 2b includes the theoretical prediction of the electronic ground state and 3p states. Theoretical fractional differential signal is derived assuming a case where excitation occurs in all Kinetic scores, predicted signal ΔS3p consistency between 3p (q, R +) and experiment is excellent.
2. The contribution of nuclear and electronic
Figure 3 shows the theoretical nuclear and electron donating fractional difference signal x-ray scatter. In FIG. 3a, b, the core and the electron donating a size of about 4%, which means that changes in the geometry and electronic states have a significant contribution to the scattered signal. However, the electronic 3p state signal in Figure 3b in the low q region (0-1.6 Å-1) has a special negative signals, while the contribution of the same core region is small. In contrast, the contribution of the core increases with increasing q (1.7-2.5 Å-1), an electron donating only gives a small negative signal region. Compared with the HOMO, which is separated from the diffusion properties of 3p-Rydberg track concerned. Proportional to the square of the number of scattered electrons in the molecule is small signal values q → 0 at the momentum transfer. FIG. 3b as molecular ion signal q → 0 down to -4.5%, andCHD 44 corresponding to the electron-1 is cleared. It largely q> 3p 1.0 Å-1 state signal remain parallel, which means that the signal is mainly controlled by 3p loss of electrons in the core of the molecule. In q> 3.5 Å-1, the ion and electron donating 3p state is substantially the same, indicating that this region is mainly affected by the core electronics. Finally, FIG. 3a 3p comprising a difference signal calculation in two geometries. The difference is only 0.1%, at least other than the effect of an order of magnitude. This indicates that the electron donating 3p states almost independent of the geometry of the molecule, and indicate the time evolution Rydberg scattered signal will be appreciated by the nucleation kinetics plus approximately constant due to the contribution of electrons.
In summary, the authors demonstrated that the non-resonant ultrafast x-ray scatter electron density variation can be solved transitions between electronic states caused. Experimental Discussion CHD when gas molecules from the ground state to a lower energy electron beam excitation of Rydberg electronic rearrangements. Ultrafast x-ray scattering will become a powerful and versatile tool for chemistry research. Accurate measurement of the ground state and the excited electron density of states, for electronic structure theory provide important reference to deepen the understanding of the chemical bonds during the transition electron distribution evolution, and to foresee possible to monitor the experiment the chemical reaction process of molecular structure and electronic change simultaneously, providing unprecedented insight into chemical kinetics. The full text link: https: // www.nature.com/articles/s41467-020-15680-4 reports: https://www.chemistryworld.com/news/first-glimpse-caught-of-how-a-molecule- changes-when-it-absorbs-light / 4011710.article