Weak-field few-femtosecond VUV photodissociation dynamics of water isotopologues

We present a joint experimental and theoretical study of the VUV-induced dynamics of ${\mathrm{H}}_2\mathrm{O}$ and its deuterated isotopologues in the first excited state ($\tilde{A}{}^1{B}_1$) utilizing a VUV-pump VUV-probe scheme combined with $ab$ initio classical trajectory calculations. 16-fs VUV pulses centered at 161 nm created by fifth-order harmonic generation are employed for single-shot pump-probe measurements. Combined with a precise determination of the VUV pulses' temporal profile, they provide the necessary temporal resolution to elucidate sub-10-fs dissociation dynamics in the 1+1 photon ionization time window. Ionization with a single VUV photon complements established strong-field ionization schemes by disclosing the molecular dynamics under perturbative conditions. Kinetic isotope effects derived from the pump-probe experiment are found to be in agreement with our by ab initio classical trajectory calculations, taking into account photoionization cross sections for the ground and first excited state of the water cation.

Figure 1

Sketch of the optical setup used to perform the single-shot pump-probe experiment.

Figure 2

Ion microscope image and delay-dependent ion signal originating from nonresonant two-photon ionization of Kr. The microscope image (a) shows the averaged spatially resolved ${\mathrm{Kr}}^{+}$ ion yield for 3000 laser pulses. The averaged delay-dependent ${\mathrm{Kr}}^{+}$ ion yield (b) corresponds to the VUV pulse intensity autocorrelation, which yields an instrument response function with a FWHM of (23.1 $\pm$ 1.4) fs. From the single-shot intensity autocorrelation for each laser pulse (c), the root-mean-square uncertainty of its FWHM (d) can be deduced.

Figure 3

(a) Cuts through the potential energy surfaces for the ground and first excited electronic states of ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{H}}_2{\mathrm{O}}^{+}$ calculated at the MR-CISD(8,7) and MR-CISD(7,7) level, respectively (solid lines), and for the $\tilde{A}{}^1{B}_1$ state of ${\mathrm{H}}_2\mathrm{O}$ calculated with CASSCF(6,4) (dashed line). The dissociation coordinate is obtained by changing the length of one of the O-H bonds (${\mathrm{R}}_{\mathrm{OH}}$) and maintaining the other O-H bond length and the angle HOH at the equilibrium values of 0.958 Å and $104.47{}^{\circ }$, respectively. ${\mathrm{R}}_{\mathrm{OH}}$ = 0 corresponds to the equilibrium bond length. (b) The dominant configurations for the $\tilde{A}{}^1{B}_1$ state of a neutral water molecule, and $\tilde{X}{}^2{B}_1$ and $\tilde{A}{}^2{A}_1$ states of the cation and their corresponding CI coefficients at the equilibrium geometry (W) are obtained from the MR-CISD calculation.

Figure 4

Calculated time evolution of the vertical excitation energy from the $\tilde{A}{}^1{B}_1$ state to the $\tilde{X}{}^2{B}_1$ state (blue) and from the $\tilde{A}{}^1{B}_1$ state to the $\tilde{A}{}^2{A}_1$ state (orange) during the dissociation for (a) ${\mathrm{H}}_2\mathrm{O}$, (b) $\mathrm{HDO}$, and (c) ${\mathrm{D}}_2\mathrm{O}$ and the corresponding time windows in which ionization from $\tilde{A}{}^1{B}_1$ to $\tilde{X}{}^2{B}_1$ and $\tilde{A}{}^2{A}_1$ with a single photon of 7.7 eV energy (gray line) is possible. The error bars represent the root-mean-square width of the energy distribution for a given time.

Figure 5

Ion microscope images and delay-dependent ion signals for ${\mathrm{H}}_2\mathrm{O}$, $\mathrm{HDO}$, and ${\mathrm{D}}_2\mathrm{O}$. The microscope image (a) shows the averaged spatially resolved ion yield of the different isotopologues for 3000 laser pulses. The delay-dependent ion signal (b) of each isotopologue is shown in comparison to the instrument response function (blue) recorded in the corresponding measurement series and fitted with a Gaussian function (red). Depending on the degree of deuterization, an increased FWHM is observed in respect to the instrument response function, indicating a kinetic isotope effect on the excited-state dynamics of the molecule.