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.
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Photofragment-ion imaging is a powerful experimental technique used in spectroscopy and chemical physics to study the dynamics of molecular fragmentation processes. This method allows researchers to visualize and analyze the products of a photodissociation event—where a molecule absorbs light energy and breaks apart into smaller fragments.