A new Coulomb breakup experiment was performed for ${}^7\mathrm{Li}$ with an improved experimental technique and theoretical treatment. Energy spectra of $\alpha$ particles and tritons were examined to find the signature of post-Coulomb acceleration in the breakup of ${}^7\mathrm{Li}$ at $E_{\alpha t}=0.\mathrm{}$ The data revealed the delayed nature of nonresonant breakup of astrophysical relevance that stems from quantum tunneling. Semiclassical discussions are presented of the lifetime of continuum states in ${}^7\mathrm{Li}$ and distortion of relative kinetic energies between $\alpha$ and t by post-Coulomb acceleration. Dynamical calculations of Coulomb breakup were performed by solving a time-dependent Schrödinger equation. A simple potential model of ${}^7\mathrm{Li}$ was employed. The dynamical calculations reasonably reproduced experimental cross sections for both resonant and nonresonant breakup with two key ingredients: higher order effects and mixture of $E1\mathrm{}$ and $E2\mathrm{}$ multipoles. Considering the dominant role of the first-order $E1\mathrm{}$ nature in adiabatic Coulomb breakup, cross sections in the $v_{\alpha }>~v_t$ branch at $7°$–$15°$ for ${}^{64}\mathrm{Zn}$ and ${}^{90}\mathrm{Zr}$ were used to deduce astrophysical S factors $S\left(E\right)\mathrm{}$ for $t(\alpha ,\gamma {)}^7\mathrm{Li}.$ They exhibit a moderate energy dependence at small energies. The strongly energy-dependent $S\left(E\right)\mathrm{}$ resulted from the previous Coulomb breakup experiment based on cross sections with $v_{\alpha }<~v_t;$ they are most likely Coulomb distorted and are revised in the present work.

• Received 13 September 2000

DOI:https://doi.org/10.1103/PhysRevC.63.035801