Dissociation and ionization of an ${\text{HD}}^{+}$ beam induced by intense 395-nm ultrashort laser pulses

The majority of research on intense $\left(\sim {10}^{14}\text{W}/{\text{cm}}^2\right)$ ultrashort $\left(<100\text{fs}\right)$ laser-molecule interactions has been focused on studies of ${\text{H}}_2^{+}$ fragmentation, mainly due to its elemental structure. So far the bulk of this work has been conducted using near infrared light while studies at shorter wavelengths are comparatively scarce. We report a detailed investigation of the interaction of 395 nm, 40 fs pulses with an ${\text{HD}}^{+}$ molecular-ion beam, measured using a coincidence three-dimensional momentum imaging technique. This allows us to clearly discriminate dissociation and ionization channels. From the kinetic energy and angular distributions, insight is gained into the intensity dependence of the main breakup processes. We observe the onset of above-threshold dissociation above ${10}^{14}\text{W}/{\text{cm}}^2$, a higher intensity than required for 790 nm due to a smaller transition dipole moment for the low vibrational states probed at 395 nm. Ionization spectra display structure consistent with the above-threshold Coulomb explosion mechanism that we have proposed [B. D. Esry et al., Phys. Rev. Lett. 97, 013003 (2006)].

Figure 1

(Color online) Schematic of the laser-ion-beam interaction region and detection setup. The beams cross within a region of applied electric field that accelerates the charged fragments toward the detector so that the ${\text{H}}^{+}$, ${\text{D}}^{+}$, and neutral fragments can all be distinguished by their flight time to the detector.

Figure 2

(Color online) Raw coincidence map of the flight time of the first particle to arrive at the detector $\left({\text{t}}_1\right)$ plotted against the flight time of the second $\left({\text{t}}_2\right)$, showing islands due to the different breakup channels as labeled. Note that the sharp lines are real event pairs that conserve momentum while the surrounding blobs are random coincidences between uncorrelated particles which mostly are eliminated after data processing (see text). The inset on the right shows the 1D time of flight where the ${\text{H}}^{+}$, ${\text{D}}^{+}$, and neutral fragments are all clearly separated.

Figure 3

(Color online) (a) Diabatic Born-Oppenheimer potentials of ${\text{HD}}^{+}$, dressed by net absorbed number of photons, $n\omega$, at 395 nm. The horizontal lines are the calculated unperturbed lowest vibrational levels. [(b)–(d)] Same as (a) with additional curves (thinner red lines) for the adiabatic Floquet potentials at (b) ${10}^{13}$, (c) ${10}^{14}$, and (d) $2\times {10}^{14}\text{W}/{\text{cm}}^2$. [(e) and (f)] Same as (b) for 790 nm at (e) $3\times {10}^{13}$ and (f) $2\times {10}^{14}\text{W}/{\text{cm}}^2$. The arrows and labels denote significant pathways of importance for the discussion in the text.

Figure 4

(Color online) Plots (a–c): Measured KER-$\text{cos}\theta$ distributions for the dissociation of ${\text{HD}}^{+}$ to ${\text{H}}^{+}+\text{D}$ using 395 nm, 40 fs pulses at intensities (a) $2\times {10}^{14}$, (b) $1.3\times {10}^{14}$, and (c) ${10}^{14}\text{W}/{\text{cm}}^2$. Plot (d): IDS for the intensity range ${10}^{14}–2\times {10}^{14}\text{W}/{\text{cm}}^2$, i.e., plot (c) subtracted from plot (a). Plot (e): angular distribution corresponding to the IDS spectra in (d) for KER ranges indicated. The curves are fits to the data. Error bars in (e) are the statistical error of the data.

Figure 5

(Color online) KER distributions, integrated for all angles, for the dissociation of ${\text{HD}}^{+}$ to $\text{H}+{\text{D}}^{+}$ at intensities indicated in units of $\text{W}/{\text{cm}}^2$, normalized to their peak counts. The dashed curve is the intensity-difference spectrum for the intensity range ${10}^{14}–2\times {10}^{14}\text{W}/{\text{cm}}^2$. Error bars are the statistical error of the data.

Figure 6

(Color online) [(a) and (b)] KER-$\text{cos}\theta$ plots for 395 nm at intensities $3\times {10}^{13}$ and $2\times {10}^{14}\text{W}/{\text{cm}}^2$, respectively. [(c) and (d)] Same as (a) and (b), respectively, for 790 nm.

Figure 7

(Color online) Dressed-states diagram of ${\text{HD}}^{+}$ at 395 nm wavelength including both the dissociation (solid) and ionization (dashed) curves. UVS, BS, and ATD indicate ionization pathways that are initiated by dissociation of the UVSs and the processes of BS and ATD (see text).

Figure 8

(Color online) KER distributions for ionization of ${\text{HD}}^{+}$ to ${\text{H}}^{+}+{\text{D}}^{+}$ at (a) $2\times {10}^{14}$ and (b) $1.3\times {10}^{14}\text{W}/{\text{cm}}^2$. The tick marks with labels indicate expected positions of peaks due to different ionization pathways, see text. The data (solid points) have been fitted with Gaussian peaks (dashed curves) as discussed in the text. The solid curves are the sum of these fits. The error bars are the statistical error of the data.