Let L=−Δ+μ be the generalized Schrödinger operator on Rn, n≥3, where μ≢0 is a nonnegative Radon measure satisfying certain scale-invariant Kato conditions and doubling conditions. Based on Shen's work for the fundamental solution of L in [23], we establish the following upper bound for semigroup kernels , associated to e−tL,
where ht(x)=(4πt)−n/2e−|x|2/(4t), and dμ(x,y,t) is some parabolic type distance function associated with μ. As a consequence,
where m(x,μ) is some auxiliary function associated with μ . We then study a Hardy space a0788802163cd293a8af073"> by means of a maximal function associated with the heat semigroup e−tL generated by −L to obtain its characterizations via atomic decomposition and Riesz transforms. Also the dual space BMOL of a0788802163cd293a8af073"> is studied in this paper.