Infrared pumping and its effect on the excitation of HCN molecules can be important when using rotational lines of HCN to probe dense molecular gas in galaxy nuclei .
We report the first extragalactic detection of ( sub ) millimeter rotational lines of vibrationally excited HCN , in the dust-enshrouded nucleus of the luminous infrared galaxy NGC 4418 .
We estimate the excitation temperature of \mbox { $T _ { vib } $ } \approx 230 K between the vibrational ground and excited ( v _ { 2 } =1 ) states .
This excitation is most likely due to infrared radiation .
At this high vibrational temperature the path through the v _ { 2 } =1 state must have a strong impact on the rotational excitation in the vibrational ground level , although it may not be dominant for all rotational levels .
Our observations also revealed nearly confusion limited lines of CO , HCN , HCO ^ { + } , H ^ { 13 } CN , HC ^ { 15 } N , CS , N _ { 2 } H ^ { + } , and HC _ { 3 } N at \lambda \sim 1 mm .
Their relative intensities may also be affected by the infrared pumping .