Negative-ion mass spectrometric study of ion-pair formation in the vacuum ultraviolet. VI. CH3X→X−+CH+3 (X=F, Cl, Br)

Abstract

Ion-pair formation from photoexcited halomethanes, CH3X*→X−+CH+3 (X=F, Cl, Br) has been studied by measuring photodissociation efficiency curves of X− using synchrotron radiation in the 9.9–27.5 eV photon energy range. A new spectral feature is observed in each of the curves near the threshold for the removal of an na1 electron from CH3X (n=4, 6, and 8, respectively, for CH3F, CH3Cl, and CH3Br ). This feature, composed of two or three peaks in each case, is interpreted as resulting from photoexcitation to the Rydberg states converging to CH3X+(C̃ 2A1), which then predissociate into ion pairs through avoided potential energy surface crossings. The interpretation is based on the results of the inner-shell electron energy loss study by Brion and co-workers and the photoabsorption study by Hochmann and co-workers. Peak features are also observed in the X− efficiency curves near the ionization threshold for CH3X+(X̃ 2E). The origins of these peaks are also discussed.

About the photoionization of methyl bromide (CH3Br). Photoelectron and photoionization mass spectrometric investigation

“…Referring to the 2 2 E energy in CAS (9,6) at the neutral molecule geometry, the 2 A" minimum and the 2 A' lie at about 1.2-1.4 eV below the apex of the double cone, which corresponds to a much larger Jahn-Teller effect than for the CH 3 Br+ ( X 2 E) ground state (0.036 eV [2]). The 2 A' and 2 A" components geometries of the 2 2 E state are characterized by a larger C-Br bond (≈5%) but mostly by larger C-H bond lengths, either C-Hl ( 2 A' (CI)) or C-H2 ( 2 A" (min)) (see Table 6).…”

“…In a negative-ion mass spectrometric study of the ion-pair formation, the production of X -ions has been investigated in the CH 3 X (X = F, CI, Br and I) molecules [6]. Owing to the lack of intensity of the photon beam in this experiment, no Br -ions have been detected below the 15 eV photon energy range.…”

Accurate calculation of the ionization energies of the chlorine lone pairs in 1,1,1-trifluoro-2-chloroethane (HCFC-133a)

“…For instance, in the case of CH 3 Cl, it is a matter of debate as to whether one should use carbon-type Rydberg labels [22] or chlorine-type Rydberg labels [23]. These two labels yield n=3 and 4 for the low-lying Rydberg states, respectively.…”

Two Studies in Gas-Phase Ion Spectroscopy

“…These data are available in the literature for CF 4 [41], SF 6 [42,43], CH 4 [44], CH 3 X (X = F, Cl, Br) [45], C 2 H 4 [46], C 2 H 6 [47,48], C 3 H 8 [47], and the chlorofluoromethanes (CFCl 3 , CF 2 Cl 2 , CF 3 Cl) [49]. The present work is in excellent agreement with these earlier studies and in most cases the quality and quantity of data obtained have improved.…”

“…Results have shown that in most cases, if charge transfer is energetically allowed, it will occur and is usually the dominant reaction channel [34][35][36][37][38][39][40][41][42][43][44][45]. The same experiments have shown that some cations and molecules do not react when the energetics do not allow for charge transfer.…”