Excited NO2 molecule

Figure 8 Illustration of the various excitation schemes used to study the unimolecular dissociation of NO2. The dissociation laser with wavelength Apu excites strongly mixed states near the dissociation threshold. The population of excited NO2 molecules is monitored by excitation to a higher electronic state with a second laser (Apr) and collecting the laser-induced-fluorescence (LIF) intensity, /lif- The decay of /lif as a function of the delay time, At, between pump and probe laser provides direct information about the dissociation of highly excited NO2. Alternatively, the appearance of the products can be monitored by tuning a specific transition of NO, Alif, and detecting the LIF signal.

An exception from the studies of non-photochemical processes is the theoretical work on NO2 photolysis [2]. Stochastic simulation of NO2 (X Ai) O( Pj) + NO (X2ria) photodissociation in the wavelength range between 348 and 379 nm lends no support to the hypothesis that vibrationally excited NO2 molecules should give a distinct contribution to the quantum yield in the vicinity of the nominal dissociation wavelength. 

The reaction forms a characteristic luminescence due to an electronically excited NO2 molecule decaying to a lower energy state. The intensity of the luminescence is proportional to the concentration of NO. The light emission is detected by a photomultiplier tube, which generates a proportional electronic signal. 

As seen in Fig. 4.9, the absorption spectrum of NO2 extends over the whole range of the visible region, and the formation rate of excited NO2 molecules by the absorption of solar radiation beyond the wavelength longer than the dissociation threshold is large. Most of the non-dissociative electrrmic excited NO2 molecules undergo the processes. 

NO2 is one of the few molecules for which the dependence of the dissociation rate on the rotation of the excited complex has been investigated. In a double-resonance experiment. Bezel et al. [276] first excited NO2 with an IR photon to a particular rovibrational state, (1, 0,1 J, Ka), and a second laser with fixed frequency promoted the prepared molecules into the continuum. Here, J is the total rotational angular momentum quantum... 

The results indicate that the NO2 molecule captures an electron to form N02. The N02 ion may then be excited to the metastable Bi state. More likely, the N02 ion is formed directly in the electron capture process by NO2. In either instance, the excited Bi state can add another electron to form the. .. (6ai) (2b2) (5bi) A2 metastable state of N02 . Huber, Cosby, Peterson and Moseley have recently studied the photodetachment and de-excitation of vibra-tionally excited N02 that survives several hundred collisions with O2. The electronically excited metastable N02 also is expected to have a relatively long lifetime. 

According to Bufalini (158) Oil A) produced partially by energy transi from the electronically excited NO2 to O2 molecules, postulated by Frank 1 wicz and Berry (372a), does not contribute to air pollution. 

When CO2 is added to the expansion, the clustering of CO2 with NO competes with the dimerization, causing a decrease in the total signal. This reduction of the signal, attributable to NO/CO2 clusters, points to the retarding effect the polyatomic spectator has on the formation of electronically excited NO2. It can be due either to a decrease in the total cross section for reaction or to quenching of the channel that produces the electronically excited product. The same results were obtained when SO 2 was used instead of CO2. A similar effect was observed in the reaction of CO with oxygen, where two CO2 molecules attached to CO dramatically reduced the production of the electronically excited C02. ... 

The selective excitation of single upper levels, which is possible in molecular beams with sufficiently good collimation, results even in polyatomic molecules in astonishingly simple fluorescence spectra. This is, for example, demonstrated in Fig. 1.56 for the NO2 molecule, which is excited at a fixed wavelength Xl = 592 nm. The fluorescence spectrum consists of readily assigned vibrational bands that are composed of three rotational lines (strong P and R lines, and weak Q lines). 

In larger molecules the phase-coherence time of excited levels may be shorter than the population lifetime because of perturbations between closely spaced levels of different electronic states, which cause a dephasing of the excited-level wave functions. One example is the NO2 molecule, where the width of the Hanle signal turns out to be more than one order of magnitude larger than expected from independent measurements of population lifetime and Landd factors [851, 852]. This discrepancy is explained by a short intramolecular decay time (dephasing time), but a much larger radiative lifetime [853]. 

A presentation of direct experimental evidence for the initial fast decompositions of the more powerful mihtary explosives, HMX, RDX and TNAZ, as well as of a non-explosive molecule, dimethyl nitramine (DMNA), is a topic of paper [55]. Experimental data indicate that electronic excitation, in the gas phase, of all the nitramines studied, with 248 mn, 15 nanosecond pulses from the excimer laser leads to the formation of electronically excited NO2 ( 62), resulting from the primary N - N bond scission in the electronically... 

A comparison of decomposition of electronic excited nitro group containing molecules (C-NO2, N-NO2, O-NO2) has been characterized by using nitromethane, dimethylnitramine and isopropyl nitrate as examples. In all of the cases, the main process appeared to involve cleavage to yield excited NO2 that then further produced NO. The largely used N-nitrated insecticides thiamethoxam, clothiadin and imidacloprid have been photochemically studied. ... 

However, if the rate constants of the reaction or energy transfer between the electronically excited NO2 and other atmospheric molecules are large enough, the possibility of such processes cannot be denied. An energy transfer process. 

Particularly for polyatomic molecules with their complex visible absorption spectra, the reduction of the Doppler width is essential for the resolution of single lines [10.4]. This is illustrated by a section from the excitation spectrum of the NO2 molecule, excited with a single-mode tunable argon laser around X = 488 nm (Fig,10.4). For comparison the same section of the spectrum as obtained with Doppler-limited laser spectroscopy in an... 

Schoen et al. (1982) observed the picosecond timescale photodecomposition of nitromethane using laser-induced fluorescence of ground state NO2 molecules. From their experiments at low pressures, the approximate quantum yields of process (la) at wavelengths 238, 254, 296, and 337 nm can be estimated to be < a 0.2,0.7 0.3, 0.7, and 0.3, respectively, with quantum yields of electronically excited NO2 formed in (Ib), ib 0.8, 0.3, 0.0, and 0.0, respectively. However, extrapolation of these findings to conditions of the lower troposphere is not possible since the extent of quenching of the excited CH3NO2 molecules is unknown, but this is expected to occur to some extent under tropospheric conditions. 

The sample is burned in oxygen at 1000°C. Nitrogen oxide, NO, is formed and transformed into NO2 by ozone, the NO2 thus formed being in an excited state NO. The return to the normal state of the molecule is accompanied by the emission of photons which are detected by photometry. This type of apparatus is very common today and is capable of reaching detectable limits of about 0.5 ppm. 

NO2 refers to the excited nitrogen oxide molecule. These molecules can decay by emission of light of wavelengths longer than 600 nm.- ... 

---