Alkali-metal hydride cations $X{\mathrm{H}}^{+}$ ($X=\text{Na}$, K, Rb, Cs) are potentially important in interstellar chemistry and early cosmological environments, yet their photodissociation properties remain poorly characterized. We present a systematic study of their photodissociation cross sections based on high-accuracy ab initio electronic structure calculations. The cross sections are computed under the assumption of local thermodynamic equilibrium, covering photon wavelengths from 50 nm up to the dissociation threshold, and include transitions from most rovibrational levels in the ground electronic state $1^2{\mathrm{\Sigma }}^{+}$. Prominent shape resonances are identified near the Lyman $\alpha$ line, significantly modulating the photodissociation behavior. In particular, for ${\mathrm{CsH}}^{+}$, strong contributions from the $2^2\mathrm{\Pi }$ and $4^2{\mathrm{\Sigma }}^{+}$ states are found in the spectral region between the Lyman $\alpha$ line and the Lyman limit. These results provide critical theoretical data for the spectral characterization of alkali-metal hydride cations and offer valuable input for astrophysical modeling of UV-driven processes and early-universe chemistry.

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• Molecular dissociation
• Molecular spectra
• Photodissociation
• Potential energy surfaces
• Rotational states
• Vibrational states
• X-ray lasers
• Ab initio molecular dynamics
• Schroedinger equation