Fragmentation of multiply charged hydrocarbon molecules C${}_n$H${}^{q+}$ ($n$ ≤ 4, $q$ ≤ 9) produced in high-velocity collisions: Branching ratios and kinetic energy release of the H${}^{+}$ fragment

Fragmentation branching ratios for channels involving H${}^{+}$ emission and associated kinetic energy release of the H${}^{+}$ fragment [KER(H${}^{+}$)] have been measured for multicharged C${}_n$H${}^q$${}^{+}$ molecules produced in high velocity (3.6 a.u.) collisions between C${}_n$H${}^{+}$ projectiles and helium atoms. For CH${}^q$${}^{+}$ ($q$ ≤ 4) molecules, measured KER(H${}^{+}$) were found well below predictions of the simple point charge Coulomb model (PCCM) for all $q$ values. Multireference configuration interaction (MRCI) calculations for ground as well as electronic excited states were performed which allowed a perfect interpretation of the CH${}^q$${}^{+}$ experimental results for low charges ($q$ = 2–3) as well as for the highest charge ($q$ $=$ 4). In this last case we could show, on the basis of ionization cross sections calculations and experimental measurements performed on the same systems at slightly higher velocity (4.5 a.u.), the prominent role played by inner-shell ionization followed by Auger relaxation and could extract the lifetime of this Auger relaxation giving rise to the best agreement between the experiment and the calculations. For dissociation of C${}_2$H${}^q$${}^{+}$ and C${}_3$H${}^q$${}^{+}$ with the highest charges ($q$ ≥ 5), inner-shell ionization contributed in a prominent way to the ion production. In these two cases it was shown that measured KER(H${}^{+}$) were in good agreement with PCCM predictions when those were corrected for Auger relaxation with the same Auger lifetime value as in CH${}^{3+}$.

Figure 1

Schematic view of the experimental setup.

Figure 2

(a) H${}^{+}$ image recorded onto the PSD detector following C${}_2$H${}^{2+}$ → C${}^{+}$|C|H${}^{+}$ dissociation; $X$ and $Y$ axis are, respectively, in the horizontal and vertical plane; dimensions along $X$ and $Y$ are in channels (1 channel $=$ 0.167 mm). (b) KER(H${}^{+}$) distribution extracted from the fit of (a) using a single Gaussian distribution (dotted line) or two half Gaussians of different widths (solid line), see text.

Figure 3

Measured ionization cross sections of CH${}^{+}$ (open squares), C${}_2$H${}^{+}$ (full squares), C${}_3$H${}^{+}$ (open triangles), and C${}_4$H${}^{+}$ (full triangles) in collisions with He at $v_p$ $=$ 4.5 a.u.; lines are to guide the eye.

Figure 4

Comparison between measured and calculated ionization cross sections in $v_p$ $=$ 4.5 a.u. C${}_3$H${}^{+}$-He collisions. Black dots: experimental results; solid line: results of the IAE model (see text); dashed line: results of the IAE model without autoionization.

Figure 5

Comparison between measured and calculated KER(H${}^{+}$) in dissociation of C${}_n$H${}^q$${}^{+}$. (a) Dissociation of CH${}^q$${}^{+}$ into C${}^{(\mathrm{q}-1)+}$-H${}^{+}$ channels. (b) Dissociation of C${}_2$H${}^q$${}^{+}$ into C${}^{+}$|C${}^{+}$|H${}^{+}$, C${}^{2+}$|C${}^{+}$|H${}^{+}$, C${}^{2+}$|C${}^{2+}$|H${}^{+}$, C${}^{3+}$|C${}^{2+}$|H${}^{+}$, and C${}^{3+}$|C${}^{3+}$|H${}^{+}$ channels for $q$ $=$ 3, 4, 5, 6, and 7, respectively. (c) Dissociation of C${}_3$H${}^q$${}^{+}$ into C${}^{+}$|C${}^{+}$|C${}^{+}$|H${}^{+}$, C${}^{2+}$|C${}^{+}$|C${}^{+}$|H${}^{+,}$ and C${}^{2+}$|C${}^{2+}$|C${}^{+}$|H${}^{+}$ channels for $q$ $=$ 4, 5, and 6, respectively. Open triangles refer to cases where inner-shell ionization contributes in a prominent way (>50%) to the ion production. Solid lines refer to predictions of the PCCM model.

Figure 6

MRCI potential energy curves of states of CH${}^{+}$ (the two lowest curves) and CH${}^{2+}$ (the five highest curves), together with dissociation limits, relevant to interpretation of measured KER after dissociation of CH${}^{2+}$. The two vertical lines refer to the Franck-Condon region from $v$ $=$ 0.

Figure 7

MRCI potential energy curves of states of CH${}^{3+}$, together with dissociation limits, relevant to the interpretation of KER measurements following dissociation of CH${}^{3+}$. The two vertical lines refer to the Franck-Condon region.

Figure 8

MRCI calculations of potential energy curves of CH${}^{3+}$ states with one inner-shell vacancy (the four top states) together with MRCI calculations of CH${}^{4+}$ states with all ionization in the valence shells (the two bottom states) pertinent to interpretation of measured KER following dissociation of CH${}^{4+}$.

Figure 9

Monopole intensities for various final states of CH${}^{2+}$, starting from the ground state $X^1$Σ${}^{+}$ (a) or from the metastable $a^3$Π (b) of CH${}^{+}$. Energies are referenced with respect to the ground state of CH${}^{+}$.

Figure 10

Competition between dissociation and Auger relaxation: calculated variation of the KER value in dissociation of CH${}^{3+}$, as a function of the Auger lifetime of the inner-shell hole.

Figure 11

Comparison between measured KER distributions (solid and dotted curves, see Sec. 2) in C${}^{+}$-H${}^{+}$ dissociation (a) and C${}^{2+}$-H${}^{+}$ dissociation (b) and calculated KER starting from the ground state of CH${}^{+}$ (vertical black lines) or from the metastable $a^3$Π state of CH${}^{+}$ (vertical gray lines). Arrows correspond to PCCM predictions.

Figure 12

Comparison between measured KER distributions (solid and dotted curves, see Sec. 2) and calculated KER (vertical black lines) in C${}^{3+}$-H${}^{+}$ dissociation as a function of the inner-shell vacancy Auger lifetime. Arrows correspond to PCCM predictions.

Figure 13

Comparison between measured KER(H${}^{+}$) and PCCM predictions corrected from Auger with a lifetime of 20 fs (dashed line) for the dissociation of C${}_2$H${}^q$${}^{+}$, $q$ $=$ 5–7.

Figure 14

Same legend as Fig. 13 for dissociation of C${}_3$H${}^q$${}^{+}$, $q$ $=$ 5–6.