Fragmentation of ${\text{CD}}^{+}$ induced by intense ultrashort laser pulses

The fragmentation of ${\mathrm{CD}}^{+}$ in intense ultrashort laser pulses was investigated using a coincidence three-dimensional momentum imaging technique improved by employing both transverse and longitudinal electric fields. This allowed clear separation of all fragmentation channels and the determination of the kinetic energy release down to nearly zero, for a molecule with significant mass asymmetry. The most probable dissociation pathways for the two lowest dissociation limits, $\text{C}{}^{+}+\text{D}$ and $\text{C}+{\text{D}}^{+}$, were identified for both 22-fs, 798-nm and 50-fs, 392-nm pulses. Curiously, the charge asymmetric dissociation of ${\mathrm{CD}}^{2+}$ was not observed for 392-nm photons, even though it was clearly visible for the fundamental 798 nm at the same peak intensity.

Figure 1

(a) Schematic of the experimental setup. (b) Coincidence time of flight map of ${\mathrm{CD}}^{+}$ fragmentation in 22-fs, 798-nm and $6.3\times {10}^{15}$ ${\mathrm{W}/\mathrm{cm}}^2$ laser pulses, which shows distinguishable breakup channels of ${\mathrm{CD}}^{+}.$ Most of the breakup channels were measured with low voltage on the deflector, while the $\text{C}{}^{+}+\text{D}$ channel (pink label) was measured with high voltage on the deflector (see Sec. 2).

Figure 2

The simulated [(a),(e)] and measured [(b)–(d)] positions of the different ${\mathrm{CD}}^{+}$ fragments on the imaging detector employing weak [39.3 V/cm, (a)–(c)] and strong [170 V/cm, (d) and (e)] transverse electrostatic fields. The scatter plots shown in panels (b)–(d) were measured with 22-fs, 798-nm and $6.3\times {10}^{15}$ ${\mathrm{W}/\mathrm{cm}}^2$ laser pulses. The blue lines and dots are associated with deuterium fragments, while the red lines and magenta dots are associated with the carbon fragments. The Faraday cup is denoted by FC and the value of $\Delta y$ in each panel indicates the deflection of the ${\mathrm{CD}}^{+}$ beam spot (marked by FC on the detector) from the beam axis (all dimensions in the figure are in millimeters). Recall that the ion beam is pointing into the page.

Figure 3

Lowest few potential energy curves (PECs) of ${\mathrm{CD}}^{+}$ (assumed to be the same as the ones reported in literature for ${\mathrm{CH}}^{+}$ [57, 58, 59]): (a) singlet states from Ref. [58], and (b) triplet states from Ref. [59] except the $c{}^3{\Sigma }^{+}$ state, which is from Ref. [57]. Note that the energy in each panel is given relative to the lowest dissociation limit of this group of states. The computed vibrational population of the $X{}^1{\Sigma }^{+}$ and $a{}^3\Pi$ states of ${\mathrm{CD}}^{+}$ are shown as insets in panels (a) and (b), respectively.

Figure 4

The measured KER and angular distributions for $\text{C}+{\text{D}}^{+}$ [(a)–(d)] and $\text{C}{}^{+}+\text{D}$ [(e)–(h)] dissociation at the highest intensity, $6.3\times {10}^{15}$ ${\mathrm{W}/\mathrm{cm}}^2$ and $6.4\times {10}^{14}$ ${\mathrm{W}/\mathrm{cm}}^2$ for the fundamental IR (798 nm, 22 fs), and second harmonic (392 nm, 50 fs), respectively, as labeled on the panels (KER-$cos\theta$ density plot [(c)–(f)] and KER distributions integrated over all angles [(a),(b), and (g),(h)]). Note that in the former the measured data for $cos\theta >0$ were reflected about zero to reduce the impact of fragment losses, and in the latter the shaded areas about the measured distributions represent statistical errors.

Figure 5

Light-dressed-states diagram of a few potential energy curves of ${\mathrm{CD}}^{+}$ singlet states (taken from Fig. 3) relevant to the dominant dissociation pathways leading to ${\text{C}}^{+}{(}^{2}P)+\text{D}{(}^{2}S)$ $[X$ ${}^1{\Sigma }^{+}$ and $A$ ${}^1\Pi$ states—dashed lines] and $\text{C}{(}^{3}P)+{\text{D}}^{+}$ [2 ${}^1{\Sigma }^{+}$ and 3 ${}^1{\Sigma }^{+}$ states—solid lines] (see text). Dissociation pathways involving the 392 nm are denoted by $2\omega ,4\omega$, and $6\omega$, where $\omega$ is the energy of the 798-nm photon [66]. The electronic states are indicated by color as follows: $X$ ${}^1{\Sigma }^{+}$ (black), 2 ${}^1{\Sigma }^{+}$ (blue), 3 ${}^1{\Sigma }^{+}$ (red), and $A$ ${}^1\Pi$ (magenta).

Figure 6

Light-dressed-states diagram of a few potential energy curves of ${\mathrm{CD}}^{+}$ triplet states (taken from Fig. 3) relevant to the dissociation pathways leading to ${\text{C}}^{+}{(}^{2}P)+\text{D}{(}^{2}S)$ $[a{}^3\Pi$ and $c{}^3{\Sigma }^{+}$ states—dashed lines] and $\text{C}{(}^{3}P)+{\text{D}}^{+}$ $[b{}^3{\Sigma }^{-}$ and $d{}^3\Pi$ states—solid lines] (see text). Dissociation pathways involving the 392 nm are denoted by $2\omega$ and $4\omega$, where $\omega$ is the energy of the 798-nm photon [66]. The electronic states are indicated by color as follows: $a{}^3\Pi$ (black), $c{}^3{\Sigma }^{+}$ (magenta), $b{}^3{\Sigma }^{-}$ (red), and $d{}^3\Pi$ (blue).

Figure 7

Lowest few potential energy curves of ${\mathrm{CH}}^{2+}$ from Ref. [75]. Note the avoided crossing between the 2 ${}^2{\Sigma }^{+}$ and 3 ${}^2{\Sigma }^{+}$ at $R_c=19.4$ a.u. (indicated by an arrow). It is the 3 ${}^2{\Sigma }^{+}$ state which correlates to the ${\text{C}}^{2+}+\text{D}$ dissociation limit (green tic mark) [75]. The measured KER spread is marked by the horizontal red dotted lines, where the middle line signifies the average KER. The corresponding internuclear distances (hence $R_i)$, estimated assuming no KER is gained prior to ionization to the 2 ${}^2{\Sigma }^{+}$ (gray) and the 3 ${}^2{\Sigma }^{+}$ and 2 ${}^2\Pi$ (yellow) states, are marked by the shaded areas.

Figure 8

The measured spectra for CAD, namely, ${\mathrm{CD}}^{+}$ breakup into ${\text{C}}^{2+}+\text{D}$ in 22-fs pulses at 798 nm and peak intensity of $6.3\times {10}^{15}$ ${\mathrm{W}/\mathrm{cm}}^2$: (a) KER-cos $\theta$ density plot, (b) KER distribution integrated over all angles (the shaded area represents the statistical errors), and (inset) angular distribution integrated over all KER. Note that CAD is strongly aligned along the laser polarization.