This is what you're looking for:
> paste("This_", letters, sep="")
> [1] "This_a" "This_b" "This_c" "This_d" "This_e" "This_f" "This_g" "This_h"
[9] "This_i" "This_j" "This_k" "This_l" "This_m" "This_n" "This_o" "This_p"
[17] "This_q" "This_r" "This_s" "This_t" "This_u" "This_v" "This_w" "This_x"
[25] "This_y" "This_z"
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