Following dynamic nuclear wave packets in N${}_2,{\mathrm{O}}_2$, and CO with few-cycle infrared pulses

We study the evolution of nuclear wave packets launched in molecular nitrogen, oxygen, and carbon monoxide by intense 8-fs infrared pulses. We use velocity map imaging to measure the momentum of the ion fragments when these wave packets are interrogated by a second such pulse after a variable time delay. Both quasibound and dissociative wave packets are observed. For the former, measurements of bound-state oscillations are used to identify the participating states and, in some cases, extract properties of the relevant potential-energy surfaces. Vibrational structure is resolved in both energy and oscillation frequencies for the cations of oxygen and carbon monoxide, displaying the same quantum wave-packet motion in both energy and time domains. In addition, vibrational structure is seen in the dication of carbon monoxide in a situation where the energy resolution by itself is inadequate to resolve the structure.

Figure 1

Inverted momentum images for detected (top to bottom) (a) N${}^{+}$, (b) O${}^{+}$, and (c) C${}^{+}$ ions from N${}_2$, O${}_2$, and CO, respectively at the pump-probe delay of 500 fs.

Figure 2

A plot of the KER vs pump-probe delay for (a) O${}^{2+}$ and (b) N${}^{2+}$ ions. The descending curves correspond to the population of the parent and final molecular ions indicated in the legend. The dashed curves were calculated from the Coulombic model discussed in the text.

Figure 3

Partial energy-level scheme for N${}_2$, N${}_2$${}^{+}$, and N${}_2$${}^{++}$. Adapted from Refs. [23, 24, 25].

Figure 4

(a) KER vs τ for N${}^{+2}$ ions emitted at ±30° to the polarization vector. The lower panel shows a projection of the 20 (±1) eV band. (b) Power spectrum of (a). The vibrational combs for the $X$ ${}^1$Σ${}_{\mathrm{g}}^{+}$ state of N${}_2$${}^{2+}$ and the $A$ ${}^2$Π${}_{\mathrm{u}}$ state of N${}_2$${}^{+}$ are indicated.

Figure 5

(a) Density plot of the yield of O${}^{+}$ ions as a function of the KER and delay time for the low-KER region. (b) Projection of the spectrum onto the KER axis. The expected vibrational combs for the $a$ ${}^4$Π${}_{\mathrm{u}}$ state are shown for one- and two-photon absorptions from the probe beam, with the lowest vibrational quantum number being $\nu$ = 11 and 0, respectively. See text.

Figure 6

(a) Selected PECs for O${}_2$ and O${}_2$${}^{+}$. (b) A detailed view of the process, whereby the $a$ ${}^4$Π${}_{\mathrm{u}}$ state populated by the pump is pumped by a single- or double-photon absorption to the $f$ ${}^4$Π${}_{\mathrm{g}}$ state by the probe and dissociated to O${}^{+}$/O. The FC region is denoted. See text for details. Adapted from Refs. [25, 38].

Figure 7

(a) Power spectrum for the KER region below 1 eV. Expected loci for the frequency-KER for one- and two-photon absorption by vibrational states of the $a$ ${}^4$Π${}_{\mathrm{u}}$ state are shown as dots and stars, respectively. The short line segment is from Eq. (3), evaluated for $\nu$ = 11. (b) Similar to (a) but for the 1.3–2.3 eV KER range. See Table  and text.

Figure 8

The KER spectrum for the observation of C${}^{+}$ ions from CO. The vibrational states identified by Lundqvist et al.[31] are indicated in the figure. The data are from a ±15° slice perpendicular to the polarization vector.

Figure 9

(a) Partial PECs for cation and dication CO molecules. (b) Expanded schematic for the coupling of the $B$ ${}^2$Σ${}^{+}$ state to the first dissociative limit of the dication. Adapted from Refs. [30, 31, 40, 41, 42, 43].

Figure 10

(a) Higher KER region for the observation of C${}^{+}$ ions as a function of delay τ. Here, $n$ represents the order of revivals. (b) Power spectrum of (a). The solid dots are calculated from the properties of states of the dication as shown in Table . The short line segments are from Eq. (3), evaluated for $\nu$ = 1.

Figure 11

Blowup of the low-KER region of Fig. 8. The locations of the expected KER from one- and two-photon absorption from vibrational states in the $B$ ${}^2$Σ${}^{+}$ well are shown.

Figure 12

(a) Density plot of yield vs KER and τ for the low-KER region of Fig. 8. (b) Corresponding power spectrum. The short line segment is from Eq. (3), evaluated for $\nu$ = 6.

Figure 13

Calculated (a) and (c) and measured (b) and (d) KER spectra for O${}_2$${}^{+}$ as a function of (a) and (b) pump-probe delay and (c) and (d) frequency. Calculated KER spectra include dipole coupling of the $a$ ${}^4$Π${}_{\mathrm{u}}$ and $f$ ${}^4$Π${}_{\mathrm{g}}$ states by the 15-fs probe laser pulse with 3 × 10${}^{14}$-W/cm${}^2$ peak intensity. The power spectra (c) and (d) are obtained with a sampling time of 2 ps (see text).