Photodissociation of water induced by a long UV pulse and probed by high-energy-resolution x-ray-absorption spectroscopy

A UV-pump x-ray-probe (UVX-PP) experiment for probing UV initiated dissociation of the water molecule with long pulses is proposed on the basis of theoretical simulations. With long overlapping UV and x-ray pulses, we can obtain high-spectral-resolution x-ray-absorption or ionization spectra, containing detailed information about dynamics in the intermediate valence-excited state. The dynamics can be influenced with frequency detuning of the pump UV pulse, which acts as a camera shutter by regulating the duration of the UVX-PP process. Thereby, this UVX-PP setup gives access to ultrafast dynamics of the nuclear wave packet without experimentally challenging requirements of ultrashort pulses and controlled delay times. In a case study of the water molecule, we focus on the lowest UVX-PP channel ($1b_1\to 4a_1, 1a_1\to 1b_1$) where both intermediate valence-excited and final core-excited states are dissociative. The variation of the UVX-PP duration, controlled by the UV detuning, and different dispersion laws of so-called molecular and atomic bands allow one to study the dynamics of fragmentation of the water molecule in the intermediate state. A feature is that the long lifetime of intermediate valence-excited states opens a door for studies of photoinduced dissociation of polyatomic molecules with heavy fragments.

Figure 1

UV$+$x-ray pump-probe scheme exemplified for the lowest UVX-PP channel of the ${\mathrm{H}}_2\mathrm{O}$ molecule: UV transition $1b_1\to 4a_1$ [27] followed by x-ray transition $1a_1\to 1b_1$. The details of calculations of potential-energy surfaces depicted in this figure are outlined in Sec. 3.

Figure 2

Theoretical UV and x-ray-absorption profiles of ${\mathrm{H}}_2\mathrm{O}$. The experiments give the following value of the peak positions: 7.447 eV for $1b_1\to 4a_1$ UV absorption [27] and 534 eV for $1a_1\to 4a_1$ x-ray absorption [37].

Figure 3

Theoretical UVX-PP spectra (4) of ${\mathrm{H}}_2\mathrm{O}$ for different detuning ${\mathrm{\Omega }}_{\mathrm{UV}}={\omega }_{\mathrm{UV}}-{\omega }_{10}+{\epsilon }_0$. One can see the anti-Raman dispersion (8) for the molecular band and nondispersive for the atomic peak, which vanishes for large $|{\mathrm{\Omega }}_{\mathrm{UV}}|$. Guidelines for anti-Raman dispersion (blue) and nondispersive (red) are included. The UVX-PP spectra show the gradual quenching of the nondispersive (8) atomic peak (red) with an increase of $|{\mathrm{\Omega }}_{\mathrm{UV}}|$ due to shortening of the duration $\tau$ (6). When the UVX-PP process becomes fast we have an almost instantaneous transition from ground to final state, $0\to 2$. Due to this, the dispersive molecular (blue) band (8) converges to the broadband which mimics the shape of x-ray-absorption spectroscopy $1a_1\to 4a_1$ shown in Fig. 2.