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A vibrational frequency-state residence time is the duration a probe remains in a defined frequency environment before leaving it. It differs from the time over which the probe's frequency retains statistical memory, and from a vibrational relaxation lifetime describing excitation loss.

For a state with constant exit rate $k$, let $S(t)$ be the probability of no exit by time $t$. The small-interval exit model givesThe residence-time density is $f(t)=k{e}^{-kt}$, with mean $1/k$ and variance $1/k^2$. The exponential distribution is memoryless: $S(t+s)/S(t)=e^{-ks}$. These conclusions require a constant-rate model, rather than following from the name of a spectral state alone.

For a two-state continuous-time Markov chain with A-to-B rate $a$ and B-to-A rate $b$, the mean residence times are $1/a$ and $1/b$. The normalized two-state vibrational frequency memory instead decays as $e^{-(a+b)T}$. Returning after an exit contributes to occupancy and correlation but not to survival without an exit.

An original symmetric example uses $a=b=k=0.25\mathrm{p}{\mathrm{s}}^{-1}$. Mean residence time is $4\mathrm{ps}$, while frequency-memory time is $2\mathrm{ps}$. Starting in A, the probability of being in A after $T$ iswhereas no-exit probability is $e^{-kT}$. At $T=2\mathrm{ps}$ these are about $0.684$ and $0.607$.

This produces vibrational residence-time sampling bias if a same-state observation is treated as proof that no exit occurred. Using $-\ln P_{AA}(2)/2$ as an exit-rate estimate gives about $0.190\mathrm{p}{\mathrm{s}}^{-1}$, below the stipulated $0.25\mathrm{p}{\mathrm{s}}^{-1}$. Faster time sampling can resolve more excursions, but the estimator must still account for missed transitions and state-assignment uncertainty.

In two-dimensional infrared spectroscopy, the 2D IR waiting time separates initial frequency labeling from later response readout. Primary response theory relates such measurements to frequency memory under explicit approximations.^[ref] A fitted memory decay is therefore not automatically a directly measured single-state residence time. Nonexponential survival can require additional states, time-dependent rates or a different physical model.