Sensitivity of inner-shell photoelectron spectroscopy to non-Born-Oppenheimer and photodissociation dynamics in polyatomic molecules

Gas-phase CS₂ photoexcited at 200 nm exhibits many general features of complex photodissociation: vibrational mode coupling, internal conversion, and intersystem crossing. The molecule initially undergoes bending and symmetric stretching motion before coupling to asymmetric stretch motion leading to dissociation of atomic sulfur. In the present work, we explore the ability of time-resolved inner-shell photoelectron spectroscopy at the sulfur 2p site to investigate such complex photodynamics. 

Gas-phase CS₂ was photoexcited with a 200 nm pump pulse and ionized with a 180 eV soft X-ray probe produced by the free electron laser FLASH, inducing ionization at the S 2p site (~170 eV binding energy). Ions and photoelectrons were simultaneously collected in a double-sided velocity map imaging (VMI) instrument. The momenta of the Coulomb-exploded ions probed the molecular geometry at the time of ionization, while the measured photoelectrons yielded information about the evolving electronic and nuclear structure through shifts in the S 2p binding energy.

The low repetition rate, high count rate experimental conditions prevent the use of coincidence techniques to separate overlapping contributions to the photoelectron spectrum from the various ionization channels. To overcome this, we have employed electron-ion covariance to associate sulfur 2p binding energy shifts to particular ionization channels and molecular geometries. We find a sulfur 2p binding energy shift of ~2 eV in covariance with low-momentum S²⁺ associated with dissociated atomic sulfur, and a transient enhancement in the production of higher-momentum S⁺ ions following photoexcitation and an associated broadening of the covariance photoelectron spectrum, indicating some effect on the core-level chemical shift from either the photoexcitation or the vibrational motion prior to photodissociation. Our experimental results are supported by simulations of the neutral CS₂ photodynamics and core ionization.