Photodissociation region

Here E0 and vq are the ion beam energy and velocity, respectively and l is the flight length from the photodissociation region to the detector. An 8-mm-wide beam block is used for these measurements a narrower block results in crosstalk between the two halves of the anode. 

The second method is the laser hole-burning method. By firing the excimer laser at the parent molecular beam, the intensity of the parent molecules along the detector axis is reduced for a time equal to the pulse width (full width = 16 ns) of the laser. The reduction of the parent beam signal is due to the dissociation of the parent molecule induced by the absorption of a 193-nm photon. The laser burn hole recorded by the MCS gives an accurate measure of the velocity spread and the most probable speed (vq) of the parent molecular beam traveling from the photodissociation region to the ionizer. 

The number density of CH3SCH3 in the photodissociation region is needed to estimate the absolute cross section for process 33. This number density is calculated using a standard molecular-beam flow formula [120] and the known nozzle diameter and distance between the nozzle and the... 

In the estimation of the dissociation excimer laser flux, we assume that the mildly focused laser beam has a Gaussian beam profile [121]. The photodissociation laser spot size has also been checked by examining the burn spots on thermal papers at various distances from the focusing lens. The variation of the laser beam spot size with distance from the focusing lens is consistent with that predicted by the Gaussian beam profile. In the photodissociation region the ionization laser beam spot is smaller than the photodissociation laser beam spot. 

On the other hand, at lower temperatures, the determinations below 400 nm are complicated by the contribution of the N2O4 dimer that absorbs in this region which is also the photodissociation region of NO2. Nevertheless for wavelengths higher than 400 nm, owing to the low pressures used, we were able to obtain the NO2 cross-sections which are now available at 0.01 nm intervals [15]. A temperature effect is clearly shown for this structured region (400-500 nm). 

The excellent imaging capabilities of the Herschel space telescope have revealed, with unprecedented detail, the structure of molecular clouds and the regions where dense cores are formed. It is now clear that dense cores form within the ubiquitous filamentary structure of interstellar clouds, as localized density and column density maxima. There is a threshold in gas colunm density for dense core formation, estimated to be at extinctions of seven magnitudes from the analysis of either deep extinction images [15] or dust sub-millimetre emission maps [16], With such a threshold, the dense cores will be commOTily well shielded from the far ultra violet (FUV) radiation, with some exceptions in massive star forming regions or in photodissociation regions (PDRs) where dense molecular gas becomes directly exposed to FUV radiation. 

State of the art models of photodissociation regions today involve hundreds of chemical species, thousands of chemical reactions, and tens of physical and chemical processes in order to accurately describe the physical and chemical structure of a piece of gas illuminated by FUV photons. This degree of sophistication cannot yet be implemented in 3D dynamical models. Most PDR models are therefore steady-state and one dimensional, with ongoing developments towards simplified three... 

Photodissociation regions (PDRs) are defined as regions where the chemistry is dominated by photons. Hence the chemistry of PDR tracers must be dominated by photo-induced processes, at least indirectly. Reactive species, rapidly destroyed by reactions with H2 or abundant neutrals are therefore good tracers of the illuminated outer layers of molecular clouds. The list includes the gas coolants [CII], [OI] and [Cl], radicals like HCO, CCH or C-C3H2, as well as reactive ions like CO, HOC or CF" ". The rotational and rovibrational lines of H2 are also bright in PDRs. 

In instruments in which the voltage on the collision cell (or photodissociation region) is raised, precursor ions will be recorded at flight times (tj + equal to ... 

Figure 8 Schematic diagram of a triple-quadrupole laser-induced photodissociation mass spectrometer operated in a coaxial configuration. IS is the ion source Q1 is the precursor ion mass filter Q2 is the ion photodissociation region Q3 is the photofragmeni ion mass filter ID is the off-axis ion detector PD is a photon detector BS is a beam splitter and lA is an iodine absorption cell.

Fig. 2. CGS4 spectmm of fluorescent H2 in Hubble 12 and diemical conditions in photodissociation regions.

The Orion Nebula has been extensively studied as the nearest massive star forming region to us. As a laboratory for the study of associated phenomena, such as young stellar outflows, molecular shocks and photodissociation regions, Orion is unparalleled. This paper presents observations of the region which provide new insight into the way these processes are at work. 

In aied imaging of H2 and PAH emission in NGC 7027 and BD+30 3639 leTeals the presence of photodissociation regions in the neutral gas sorroimding these young planetary nebulae. 

Evidence for a PDR in NGC 7027 comes from comparing the H2 and PAH emission. H2 and PAH emission are coextensive outside the H H region, and the H2 loops form the outer boundary of the PAH emission. Since the PAH feature traces the penetration of FUV light into the surrounding gas this reveals the interface between ionized and molecular gas, and therefore traces the photodissociation region in NGC 7027. The same stratification of H n, PAH, H2, and CO is seen in the Orion Bar PDR (SeUgren et d. 1990). 

Here, then, are the tracers we sou t of the photodissociation regions at arcsecond resolution. 

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