Unexpected Coexistence Of Radiation And Absorption In Rotationally Excited Nitrogen Molecular Ions

We studied the dynamic evolution of the rotational states by measuring the laserlike narrowbandwidth coherent radiation of N-2(+) from N-2(+) (B-2 Sigma(+)(u)) to N-2(+) (X-2 Sigma(+)(g)). The coexistence of radiation and absorption was observed when the ratio of population in the N-2(+) (B-2 Sigma(+)(u)) and N-2(+) (X-2 Sigma(+)(g)) states is around unity. The absorption turns up on the blue side of the radiation with a time delay of 250 fs. Based on a semiclassical radiation and absorption model, it was found that the fast rotational temperature evolution, determining the population distribution on the rotational states, holds the key to understand this behaviour.