A continuous dimethylamino (CH3)(2)N +/- (DMA) radical beam is produced in situ by the pyrolysis of dimethylnitrosamine (CH3)(2)NNO at 850 (0.5 degrees C) in a double-heater inlet system on a double-chamber UPS machine-IT which was built specifically to detect transient species. The HeI photoelectron spectrum (PES) of the (CH3)2N radical is recorded for the first time. To assign the PES bands of the (CH3)2N radical, the improved density functional theory (DFT) calculation based on the Amsterdam density functional (ADF) program package has been carried out according to Ct, symmetry for the ground state of the neutral (CH3)(2)N radical and the equilibrium geometries of several ionic states of the cationic species. A sharp peak with the lowest ionization energy at 9.01 +/- 0.02 eV comes from electron ionization of the HOMO(2b(1)) of the (CH3)2N radical, corresponding to the ionization of the (CH3)2N ((XB1)-B-2) to (CH3)(2)N+(X (1)A(1)). The second band with vibration spacing 1980 +/- 60 cm(-1) comes from electron ionization of 1a(2) orbital which is a strongly bound state for the (CH3)(2)N radical, corresponding to ionization of (CH3)2N (X B-2(1)) to the B-3(1) State of (CH3)(2-) cation. The PES bands of the NO molecule clearly appears in the PE spectrum of the products pyrolyzed of the compound. The pyrolysis mechanism of the (CH3)2NNO compound is shown as follows: 850 +/- 0.5 degrees C (CH3)(2)NNO --> (CH3)(2)N + NO.