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

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, Zt=−7.59(80)×10−3 cm−1, Ft=−1.6(11)×10−4 cm−1, and Ds=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)