X-ray Scattering Research Papers - Academia.edu (original) (raw)

We use the temporal Fourier transform of time-resolved x-ray scattering, known as Fourier-transform inelastic x-ray scattering (FT-IXS), to examine the harmonic, anharmonic, and dissociative dynamics of molecular iodine. This method can directly extract dissociation velocities. OCIS codes: (320.7150) Ultrafast spectroscopy, (320.2250) Femtosecond phenomenon, (340.7440) X-ray imaging Fourier-transform inelastic x-ray scattering (FT-IXS) has enabled Mariano et al to obtain dispersion curves for germanium to arbitrary precision [1]. We have extended FT-IXS to gas-phase systems to enable easy identification of harmonic, anharmonic, and dissociative motion. The anharmonic vibrations and dissociations of molecular iodine have been studied in our previous work [2,3], which focused on doing a spatial reconstruction of the time-resolved x-ray scattering (TRXS) signal. Here we take the FT-IXS approach whereby the temporal-Fourier transform of the TRXS signal is used to obtain a dispersion plot for the system. This dispersion plot is similar to traditional spectroscopies as we now discuss with two important distinguishing factors: (1) the dispersion plot can be obtained to arbitrary frequency resolution and (2) we can identify dissociations and their velocities. The advantage of FT-IXS to the standard spatial reconstruction of time-resolved x-ray scattering is ease of computation. Spatial reconstructions are limited because time-resolved x-ray scattering has a limited Q range at present, making reconstruction of the pair-correlation function difficult and ill-defined. Temporal Fourier transforms by comparison are easy because of the ability in an experiment to scan a nearly unlimited range of pump-probe delays. The spectrum obtained by the temporal Fourier transform has a resolution given by and a maximum observable frequency given by , where is the time resolution of the experiment and is the range of the measured pump-probe delays. To compute the temporal-Fourier transforms shown in Figure 2, the fast Fourier transform of is used, where is the radially-integrated polarization-correction scattering signal from [3]. The ease of computation comes at the cost of ease of interpretation. Here we discuss the interpretation of FT-IXS from gas-phase molecules, and we highlight its most notable feature: measuring dissociation velocities. Figure 1 shows the schematic for the TRXS experiment we conducted at LCLS in 2016 [2] as well as an example difference scattering signal at the detector for an x-ray to optical delay of 120 fs. The relevant electronic states accessible by the 520 and 800 nm pump wavelengths used in the experiment are depicted in Figure 2. Figure 3 shows the FT-IXS spectra for the two wavelengths. At 800 nm, the iodine is photoexcited through a vibrational Raman process, inducing a vibrational wavepacket on the ground X state. The excited vibrational states lie in the harmonic parts of the well, so we only observe the harmonic beat frequency in the FT-IXS spectrum as expected. At 520 nm, the iodine is photoexcited the anharmonic region of the B state. In this case, we observe both the fundamental and the second harmonic of the beat frequency, consistent with the anharmonic well. Also at 520 nm, iodine is photoexcited to the dissociative B" state, which is observable in the FT-IXS spectrum as a sloped line. The slope of this line is the velocity of the dissociating iodine molecules as given by. This highlights a novel feature of FT-IXS that cannot be obtain with traditional spectroscopies: we can directly observe and fingerprint dissociation in the spectrum. Moreover, the FT-IXS spectrum extends traditional spectroscopies as FT-IXS contains not only temporal-frequency information but also spatial-frequency information that can be used to interpret dissociative and vibrational motion in the spectrum. In summary, we provide a first extension of FT-IXS to gas-phase TRXS experiments. We believe this provides a new and exciting way of analyzing TRXS experiments such as [Co(terpy)2] 2+ which exhibits clear vibrations at [4] and cyclohexadiene which has a ring opening, similar to a dissociation process, at [5]. CLEO 2018