Photochemistry sample sticks for inelastic neutron scattering

Abstract

Every material experiences atomic and molecular motions that are generally termed vibrations in gases and liquids or phonons in solid state materials. Optical spectroscopy techniques, such as Raman, infrared absorption spectroscopy, or inelastic neutron scattering (INS), can be used to measure the vibrational/phonon spectrum of ground state materials properties. A variety of optical pump probe spectroscopies enable the measurement of excited states or elucidate photochemical reaction pathways and kinetics. So far, it has not been possible to study photoactive materials or processes in situ using INS due to the mismatch between neutron and photon penetration depths, differences between the flux density of photons and neutrons, cryogenic temperatures for INS measurements, vacuum conditions, and a lack of optical access to the sample space. These experimental hurdles have resulted in very limited photochemistry studies using INS. Here we report on the design of two different photochemistry sample sticks that overcome these experimental hurdles to enable in situ photochemical studies using INS, specifically at the VISION instrument at Oak Ridge National Laboratory. We demonstrate the use of these new measurement capabilities through (1) the in situ photodimerization of anthracene and (2) the in situ photopolymerization of a 405 nm photoresin using 405 nm excitation as simple test cases. These new measurement apparatus broaden the science enabled by INS to include photoactive materials, optically excited states, and photoinitiated reactions.

Measuring Intermolecular Excited State Geometry for Favorable Singlet Fission in Tetracene

“…44 INS is measured for these model materials while photoexciting the samples in situ using a recently developed laserequipped optical excitation sample environment. 35 Figure 2 shows the INS spectrum for crystalline pentacene with the lasers off and on. The blue line shows the measurement with a 405 nm laser on, the green line shows a 520 nm laser on, and the black line is for laser off.…”

“…INS is measured for these model materials while photoexciting the samples in situ using a recently developed laser-equipped optical excitation sample environment Figure shows the INS spectrum for crystalline pentacene with the lasers off and on.…”

“…This makes INS particularly advantageous over optical vibrational spectroscopy techniques for studying organic materials . For the first time, we report in situ excited state INS measurements on organic chromophores measured using a recently developed sample environment on the indirect geometry VISION neutron spectrometer at Oak Ridge National Laboratory . The sample environment uses continuous wave 405 nm (3.06 eV) and 520 nm (2.38 eV) lasers to photoexcite samples while simultaneously measuring INS in situ.…”

“…34 For the first time, we report in situ excited state INS measurements on organic chromophores measured using a recently developed sample environment on the indirect geometry VISION neutron spectrometer at Oak Ridge National Laboratory. 35 The sample environment uses continuous wave 405 nm (3.06 eV) and 520 nm (2.38 eV) lasers to photoexcite samples while simultaneously measuring INS in situ. We measured the fractional differences to the phonon spectra between the optically excited and dark samples.…”