Branching ratios for the formation of the C(H/D)⁺, C(H/D)₂⁺ and C(H/D)₃⁺ product ions measured in the current work and the relative branching ratios for the CH⁺, CH₂⁺ and CH₃⁺ product ions measured in ref. 40–43. The branching ratios exclude the contributions from H⁺ and CH₄⁺ which could not be reliably detected in our experiments.

	C(H/D)+	C(H/D)2+	C(H/D)3+
He+ + CH4 (this work)	0.20 ± 0.03	0.72 ± 0.08	0.08 ± 0.03
He+ + CD4 (this work)	0.25 ± 0.05	0.72 ± 0.11	0.03+0.070.03
Ref. 40	0.197	0.746	0.0563
Ref. 41	0.194	0.755	0.0510
Ref. 43	0.216	0.716	0.0676
Ref. 42	0.203	0.725	0.0725