Relative Partial Cross Sections for Single, Double, and Triple Photoionization of C₆₀ and C₇₀

Abstract

Partial cross sections for the photoion formation from C(60) and C(70) were determined from the yields of singly, doubly, and triply charged ions which were measured by mass spectrometry combined with tunable synchrotron radiation at hnu = 25-120 eV. The dependence of the detection efficiencies on the mass-to-charge ratio was evaluated by using the formula proposed by Twerenbold et al. Corrections of the detection efficiency were found to be critical for obtaining accurate partial cross sections for photoionization of fullerenes. Revisions were made of the partial cross-section curves for single and double photoionization of C(60) and C(70). The curve for triple photoionization of C(70) was newly proposed. The ratios between the cross sections for double and single photoionization increase with hnu and reach saturated values of 0.78 at 85 eV for C(60) and approximately 1.3 at 100 eV for C(70). In contrast, the ratios at 120 eV between the cross sections for triple and single photoionization of C(60) and C(70) amount to 0.14 and approximately 0.38, respectively. The formation mechanism of multiply charged fullerene ions was discussed in terms of valence-electron excitation to antibonding unoccupied orbitals and/or spherical standing waves inside the cavity of a fullerene. This excitation could be followed by Spectator Auger processes and transmission of the excess electronic energy among numerous vibrational degrees of freedom.

Absolute Total Photoionization Cross Section of C60in the Range of 25–120 eV: Revisited

“…Although the dashed curve of Reinköster et al deviates upward with decreasing h below 45 eV, some of its fine structures are reproduced in the present curve: shoulders at $26 and $40 eV are seen in both the curves. 6,8) The present abs,I data agree reasonably well at h > 50 eV with 60 times the total photoabsorption cross section of a carbon atom 23) [60 Â (C) shown by squares in Fig. 3].…”

“…Nevertheless, the total cross section proposed by Kou et al 5) was not proportional to the theoretical absolute photoabsorption cross section of C 60 obtained by Colavita et al 9) Very recently, Mitsuke et al investigated the dependence of the detection efficiency of the ion detector on the mass-to-charge ratio (m=z). 8) They concluded that the relative partial cross section for double photoionization of C 60 should be about one third as large as Kou et al had expected. In this connection, Reinköster et al 6) and Juranic et al 7) have reported that the sum of the relative partial cross sections for C 60 zþ (z ¼ 1{ 3) from C 60 is consistent with the photoabsorption cross section of Colavita et al 9) To obtain an absolute photoionization or photoabsorption cross section curve of C 60 is another important issue that still remains to be settled.…”

“…We estimated the h dependence of the percentage of the second-order light by measuring the ion yield spectrum of He at h ¼ 40 {100 eV and by comparing it with the absorption cross section data of He in the literature. 8,18) For instance, the percentage at h ¼ 40 eV was found to be 22%.…”

“…However, we recently recognized that corrections for the m=z-dependence of the detection efficiency are critical to obtaining accurate cross sections for the photoionization of fullerenes. 8) In the present study, we make an evaluation of abs (C 60 zþ ) with the help of an empirical formula proposed by Twerenbold et al 8,21) First, we experimentally calculate the absolute overall detection efficiencies abs (Rg zþ ) of our mass spectrometer for singly and doubly charged photoions produced from three rare gases (Rg ¼ Ar, Kr, Xe) as…”

“…Many groups have measured the relative photoionization cross section for the production of C 60 þ from C 60 by means of mass spectrometry in the extreme UV region. [1][2][3][4][5][6][7][8] Some authors have reported 1,[4][5][6][7][8][9] that this cross section curve shows peaks and shoulders at photon energies below h $ 50 eV, reflecting the molecular character of C 60 in its valence photoionization. In contrast, the yield curve above $50 eV appears to resemble the absorption cross section of an isolated carbon atom.…”