The first excited vibrational level of the ground electronic states of nitric oxide was populated above its equilibrium value by flash photolysis of nitric oxide + inert gas mixtures, under isothermal conditions. Electronic excitation NO
2
II
(
v
= 0) +
hv
→ NO
2
Ʃ
(
v
= 0, 1, 2) was followed either by fluorescence NO
2
Ʃ
(
v
= 0, 1, 2) → NO
2
II
(
v
= 0, 1, 2...) +
hv
, or by quenching NO
2
Ʃ
(
v
= 0, 1, 2) +
M
→ NO
2
II(
v
= 0, 1, 2...) +
M
, causing a non-equilibrium population of the vibrational levels of the ground electronic states. Subsequently, the reactions NO
2
II
(
v
= 1) +
M
→ NO
2
II
(
v
= 0) +
M
and NO
2
II
(
v
= 1) + NO
2
II
(
v
= 0) → 2NO
2
II
(
v
= 1) caused a decay of the vibrationally excited molecules with time; this was followed in absorption by kinetic spectroscopy. Because of the rapidity of the last reaction, bands of NO2 II with v >1 were usually observed only in the fluorescence spectrum. In mixtures of 1 to 5 mm of NO with a large excess of nitrogen or krypton, the concentration of NO2
II
(
v
= 1) produced by the flash was of the order of 10-1 mm pressure, i. e. about the same concentration which is present in one atmosphere pressure of NO at room temperature. The absolute concentration of NO2
II
(
v
= 1) was measured accurately by plate photometry, high pressures of NO being used for calibration. The recorded probabilities of vibrational relaxation, P1-0, for NO2
II
(
v
= 1), and radii for electronic quenching,
σ
e
, by NO, N
2
, CO, H
2
O and CO
2
, are
P
1-0
σ
e
(Å) NO 3.55 x 10
-4
14 N
2
4 x 10
-7
≤ 2x 10
-2
CO 2.5 x 10
-5
0.6 H
2
O 7 x 10
-3
30 CO
2
1.7 x 10
-4
5 With the use of an analytic form for the flash duration, the entire rise and fall of the concentration of excited species was quantitatively interpreted. A very small fraction of the NO was decomposed by the flash, due either to absorption of radiation below 1900 Å or by reaction of metastable NO molecules with each other or with ground state molecules. Abnormal effects were observed in NO+ H
2
+inert gas mixtures and chemical reaction occurred.