Photodissociation separates a molecule into fragments after photon absorption. For an isolated parent with a single absorbed photon, conservation of energy givesInternal excitation energies here are relative to the parent and product ground states used to define ; is the total product translational kinetic energy in the parent's centre of mass frame. Measuring one product's kinetic energy is insufficient by itself. For two fragments of masses , conservation of momentum gives when is detected. Unknown product excitation can conceal part of the available energy.
The number of specified photochemical events per absorbed photon. A wavelength-dependent channel yield must be included when converting absorption into a chemical rate.
The first-order rate coefficient for a specified photodissociation channel. It integrates spectral actinic flux, absorption cross section and channel quantum yield. Absorption without dissociation does not contribute to that channel's rate. Its units are inverse time.
In a direct-beam model with an effective ozone cross section , removal of an overlying column multiplies the transmitted photodissociation flux by . A real photodissociation coefficient integrates wavelength-dependent cross section, quantum yield and actinic flux, including scattering and the local radiation field.
Photofragment-ion imaging detects fragment velocities by ionizing neutral products and mapping their motion onto a position-sensitive detector using an electric field. In a velocity-mapping implementation, the image radius measures transverse speed under fixed lens settings. Selecting an arrival-time slice helps extract the central plane of the three-dimensional distribution. Species identification and electronic quantum state selection are essential for turning a radius into a particular dissociation threshold.
A molecular nitrogen experiment used a pulsed nozzle and two skimmers. A tube raised the rotational temperature. Opposed vacuum-ultraviolet laser pulses excited selected molecular levels and, about later, selectively ionized excited nitrogen fragments. A microchannel plate detector, phosphor screen and camera recorded radii.[ref]
Accurate measurements of the bond dissociation energies of 14N2, 14N15N and 15N2 by Liya Lu and others. Lu, Jiang and Gao, §2, printed manuscript pp. 2–3: nozzle, tube, pump/probe and imaging readout
The measured squared radii were fitted against molecular excitation energies, referenced to the parent ground state:The intercept at zero radius gave the excited-product threshold; the fitted slope absorbed the detector scale. A wavemeter monitored the excitation source; scanning the probe covered the recoil Doppler shift. Subtracting the atomic excitation energies converted the intercept to ground-product .[ref]
Accurate measurements of the bond dissociation energies of 14N2, 14N15N and 15N2 by Liya Lu and others. §1: image-radius equation; §2: wavemeter and probe scan; §3 and Figure 4: fits; §4: atomic excitation correction
An apertured grounded plate and detector timing rejected faster competing channels. Selecting fragment electronic states distinguished nearby thresholds. Agreement of across isotopes after zero-point energy correction tested consistency. The method still requires correctly assigned levels, adequate near-threshold signal and stable imaging; a signal drop alone can reflect another channel or a barrier.[ref]
Accurate measurements of the bond dissociation energies of 14N2, 14N15N and 15N2 by Liya Lu and others. §2: shielding plate and timing; §3: state-selected channels; §4 and Table 2: isotope consistency and limitations
This measurement determines a state-resolved ground-state dissociation energy. Obtaining a finite-temperature bond-dissociation enthalpy requires the thermal increments above; obtaining a bond potential-well depth requires the zero-point energy correction. The three quantities describe different starting conditions for the same specified separation.
