Infrared studies of molecular ions. I. The ν3 band of NH4+

Abstract

Infrared spectrum of the ν₃ fundamental vibration‐rotation band of the ammonium ion NH₄⁺ has been observed. The ac discharge method recently developed by Saykally and his colleagues was used with a difference frequency laser system as the radiation source. Some 150 lines have been observed and assigned to P,q, and R branches up to J=10. An analysis of the low J transition gave the following molecular constants. ν₀−2(Bζ)′ =3342.573 (21) cm⁻¹, B′−(Bζ)′=5.523 53(70) cm⁻¹, B′−B₀=−0.049 997(19) cm⁻¹, Z_t=−7.59(80)×10⁻³ cm⁻¹, F_t=−1.6(11)×10⁻⁴ cm⁻¹, and D_s=7.2(32)×1p⁻⁵ cm⁻¹. A calculated Coriolis constant was used to obtain the ν₀ and b2d0 values as ν₀=3343.130 (56) cm and b₀=5.852 (31) cm⁻¹. These results confirm the expected tetrahedral structure of this molecular ion and give an estimated equilibrium bond length of 1.026(5) Å. (AIP)