Infrared-Induced Isomerization of Invisible Isomers (I⁵) using Nanosecond Laser Spectroscopy

Gas-phase laser spectroscopy employing nanosecond lasers has traditionally been constrained to analyzing the conformational landscape of isomers with relatively long excited-state lifetimes. The approach proposed here addresses this limitation by introducing infrared-triggered isomerization to detect previously "invisible" isomers. Molecules such as 2-acetaminophen (2-AAP), 2-hydroxyformanilide (2-HFA), and 2-aminophenol (2-AP) exhibit multiple isomeric forms, among which the ortho-substituted isomers form strong intramolecular hydrogen bonds, rendering them undetectable in conventional resonant two-photon ionization (R2PI) spectra. Infrared (IR) excitation induces intramolecular vibrational energy redistribution (IVR), enabling the transformation of these "invisible" isomers into "visible" ones. This transformation manifests as new peaks in IR-dip spectra, representing a population transfer detectable through the technique. This advancement significantly enhances the capability of laser spectroscopy to detect and analyze short-lived isomers in the gas phase, thereby broadening its applicability for structural analysis.