Multichannel dissociative ionization of ethanol in intense ultraviolet laser fields: Energy correlation between photoelectron emission and fragment recoil

We investigate multichannel dissociative ionization of ethanol in intense ultraviolet laser fields using a photoelectron-photoion coincidence momentum imaging technique. Product channels are clearly separated with detected photoions, and channel-specific photoelectron spectra exhibit multiple spectral components unambiguously assigned to four- and five-photon ionization to the electronic ground state and the electronically excited state. We measure kinetic energy distributions of fragment ions as a function of the energy of a correlated photoelectron to reveal how much energy is provided to the fragment recoil of fragment ions in the course of photoelectron emission and subsequent electronic excitation. Subsequent electronic excitation rather than photoelectron emission governs the internal energy of ethanol cations. The role of subsequent electronic excitation becomes more decisive as the laser intensity increases.

Figure 1

Channel-specific photoelectron spectra for the formation of ${\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{OH}}^{+}$ (red), ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{OH}}^{+}$ (green), ${\mathrm{CH}}_2{\mathrm{OH}}^{+}$ (blue), ${\mathrm{C}}_2{\mathrm{H}}_5^{+}$ (yellow), ${\mathrm{C}}_2{\mathrm{H}}_3^{+}$ (gray), and ${\mathrm{CH}}_3^{+}$ (brown) from the top at the laser peak intensities $I_0$ of (a) 1.5, (b) 7, and (c) 17 ${\mathrm{TW}/\mathrm{cm}}^2$. Some of spectra are multiplied as written in figures for ease of viewing. For the ${\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{OH}}^{+}$ formation, the spectra in the high-energy region assigned to five-photon ATI are vertically expanded.

Figure 2

Schematic energy-level diagram of electronic states of ${\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{OH}}^{+}$ [40] and appearance energies for ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{OH}}^{+}$, ${\mathrm{CH}}_2{\mathrm{OH}}^{+}$, ${\mathrm{C}}_2{\mathrm{H}}_5^{+}$, ${\mathrm{C}}_2{\mathrm{H}}_3^{+}$, and ${\mathrm{CH}}_3^{+}$ [41, 42, 43]. The energy levels of the electronic ground state $1^2{\mathrm{A}}^{″}$ and the electronically excited states $1^2{\mathrm{A}}^{\prime }$, $2^2{\mathrm{A}}^{″}$, $2^2{\mathrm{A}}^{\prime }$, $3^2{\mathrm{A}}^{\prime }$, and $3^2{\mathrm{A}}^{″}$ are also depicted. The adiabatic ionization energy is shown by a dotted line (red). The energies of four and five photons are shown by dashed lines. The energy is measured from the ground state of neutral ethanol ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}$. Upward arrows indicate one-photon excitation. Right arrows with the labels I–IV indicate reaction paths in terms of excitation energy. On the left side, molecular orbitals from which a photoelectron is ejected to form the respective electronic states are illustrated. These are calculated using density functional theory [B3LYP with the TZV(d,p) basis set].

Figure 3

(a–d) Correlation maps between kinetic energies of a fragment ion and a photoelectron at $I_0=1.5$ ${\mathrm{TW}/\mathrm{cm}}^2$ for the formation of (a) ${\mathrm{CH}}_2{\mathrm{OH}}^{+}$, (b) ${\mathrm{C}}_2{\mathrm{H}}_5^{+}$, (c) ${\mathrm{C}}_2{\mathrm{H}}_3^{+}$, and (d) ${\mathrm{CH}}_3^{+}$. (e–h) Temperature $k{T}_{\mathrm{ion}}$ obtained as a function of the photoelectron energy through fitting of the Boltzmann distribution to kinetic energy distributions of the fragment ions shown in (a)–(d), respectively. Fitting results of the ${\mathrm{C}}_2{\mathrm{H}}_5^{+}$, ${\mathrm{C}}_2{\mathrm{H}}_3^{+}$, and ${\mathrm{CH}}_3^{+}$ channels in the region of $E_{\mathrm{elec}}$>2.5 eV are not obtained because of insufficient event numbers.

Figure 4

Temperature $k{T}_{\mathrm{ion}}$ obtained as a function of the photoelectron energy through fitting of the Boltzmann distribution to kinetic energy distributions of fragment ions at the laser intensities of $I_0=7$ (a, b) and 17 ${\mathrm{TW}/\mathrm{cm}}^2$ (c, d) for the formation of ${\mathrm{CH}}_2{\mathrm{OH}}^{+}$ (a, c) and ${\mathrm{C}}_2{\mathrm{H}}_5^{+}$ (b, d).

Figure 5

Temperature $k{T}_{\mathrm{ion}}$ obtained as a function of the photoelectron energy through fitting of the Boltzmann distribution to kinetic energy distributions of fragment ions at the laser intensities of $I_0=7$ (a, b), and 17 ${\mathrm{TW}/\mathrm{cm}}^2$ (c, d) for the formation of ${\mathrm{C}}_2{\mathrm{H}}_3^{+}$ (a, c) and ${\mathrm{CH}}_3^{+}$ (b, d).