The generation and relaxation of light-induced metastable states at room temperature is investigated in single crystals and aqueous solutions of ${\mathrm{Na}}_2\left[\mathrm{Fe}{\left(\mathrm{C}\mathrm{N}\right)}_5\mathrm{N}\mathrm{O}\right]\bullet 2{\mathrm{H}}_2\mathrm{O}$ (SNP), using transient absorption spectroscopy. The temporal monoexponential decay after exposure to nanosecond laser pulses results from the thermal deexcitation of the metastable state named SII. The lifetime $\tau$ of SII at $T=302\mathrm{K}$ is $\tau =1.8\left(1\right)\times {10}^{-7}\mathrm{s}$ in single crystals and $\tau =1.1\left(1\right)\times {10}^{-7}\mathrm{s}$ in aqueous solutions. An Arrhenius-like behavior is verified yielding the activation energy $E_A=0.43\left(3\right)\mathrm{eV}$ and the frequency factor $Z=7\times {10}^{13}{\mathrm{s}}^{-1}$. The buildup time of the metastable state SII is faster than $1\mathrm{ns}$ and occurs via a singlet excited state. With an exposure of $0.8\mathrm{J}∕{\mathrm{cm}}^2$, 0.8(2)% of the molecules can be transferred from the ground state to SII.

• Received 18 August 2005

DOI:https://doi.org/10.1103/PhysRevB.72.195119